linked list visualisations

frontend/src/app/patterns/[slug]/page.tsx

@@ -14,11 +14,13 @@ import {
   RelatedPatterns,
 } from "@/components/patterns";
 import { PatternVisualization } from "@/components/visualization";
-import { TwoPointersVisualization, PrefixSumVisualization } from "@/components/visualizations-new";
+import { TwoPointersVisualization, PrefixSumVisualization, LinkedListVisualization } from "@/components/visualizations-new";
 import { twoSumAlgorithm } from "@/content/algorithms/two-sum";
 import { slidingWindowAlgorithm } from "@/content/algorithms/sliding-window";
 import { binarySearchAlgorithm } from "@/content/algorithms/binary-search";
 import { prefixSumAlgorithm } from "@/content/algorithms/prefix-sum";
+import { fastSlowPointersAlgorithm } from "@/content/algorithms/fast-slow-pointers";
+import { linkedListReversalAlgorithm } from "@/content/algorithms/linkedlist-reversal";
 
 interface PageProps {
   params: Promise<{ slug: string }>;
@@ -119,6 +121,10 @@ export default async function PatternDetailPage({ params }: PageProps) {
         <TwoPointersVisualization algorithm={binarySearchAlgorithm} />
       ) : slug === "prefix-sum" ? (
         <PrefixSumVisualization algorithm={prefixSumAlgorithm} />
+      ) : slug === "fast-slow-pointers" ? (
+        <LinkedListVisualization algorithm={fastSlowPointersAlgorithm} />
+      ) : slug === "linkedlist-reversal" ? (
+        <LinkedListVisualization algorithm={linkedListReversalAlgorithm} />
       ) : pattern.visualization_examples && pattern.visualization_examples.length > 0 ? (
         <Card>
           <CardHeader>

frontend/src/components/visualizations-new/algorithms/linked-list.tsx

@@ -0,0 +1,66 @@
+'use client';
+
+import { useVisualization } from '@/lib/visualizations/use-visualization';
+import type { AlgorithmDefinition, LinkedListState, LinkedListPointerState } from '@/lib/visualizations/types';
+import { VisualizationContainer } from '../core/visualization-container';
+import { LinkedListView } from '../data-structures/linked-list-view';
+
+interface LinkedListVisualizationProps {
+  algorithm: AlgorithmDefinition;
+  className?: string;
+}
+
+export function LinkedListVisualization({
+  algorithm,
+  className,
+}: LinkedListVisualizationProps) {
+  const {
+    currentStep,
+    currentStepIndex,
+    totalSteps,
+    playback,
+    controls,
+    currentPhase,
+    progress,
+  } = useVisualization(algorithm);
+
+  const { dataState } = currentStep;
+
+  // Extract linked list and pointers from dataState
+  // We store linkedLists in a custom field, falling back to empty
+  const linkedList: LinkedListState = (dataState as { linkedLists?: LinkedListState[] }).linkedLists?.[0] ?? {
+    id: 'empty',
+    nodes: [],
+  };
+
+  const linkedListPointers: LinkedListPointerState[] =
+    (dataState as { linkedListPointers?: LinkedListPointerState[] }).linkedListPointers ?? [];
+
+  return (
+    <VisualizationContainer
+      title={algorithm.title}
+      pattern={algorithm.pattern}
+      code={algorithm.code}
+      currentLine={currentStep.codeLine}
+      highlightLines={currentStep.codeHighlightLines}
+      explanation={currentStep.explanation}
+      decision={currentStep.decision}
+      phase={currentPhase}
+      variables={dataState.variables}
+      currentStepIndex={currentStepIndex}
+      totalSteps={totalSteps}
+      isPlaying={playback.isPlaying}
+      speed={playback.speed}
+      controls={controls}
+      progress={progress}
+      className={className}
+    >
+      <div className="flex flex-col items-center justify-center">
+        <LinkedListView
+          list={linkedList}
+          pointers={linkedListPointers}
+        />
+      </div>
+    </VisualizationContainer>
+  );
+}

frontend/src/components/visualizations-new/core/visualization-container.tsx

@@ -81,7 +81,7 @@ export function VisualizationContainer({
         </div>
 
         <div
-          className="flex min-h-80 items-center justify-center rounded-lg border border-border bg-surface px-8 py-4 lg:h-auto"
+          className="flex min-h-80 min-w-0 items-center justify-center overflow-x-auto rounded-lg border border-border bg-surface px-8 py-4 lg:h-auto"
           role="img"
           aria-label="Algorithm visualization"
         >

frontend/src/components/visualizations-new/data-structures/array-view.tsx

@@ -39,6 +39,30 @@ export function ArrayView({
   // Calculate total array width for proper pointer alignment
   const totalWidth = array.elements.length * elementWidth + (array.elements.length - 1) * gap;
 
+  // Check if any pointers are too close together (within 1 index) - hide values to save space
+  const hasClosePointers = pointers.length > 1 && pointers.some((p, i) => {
+    return pointers.some((other, j) => i !== j && Math.abs(p.index - other.index) <= 1);
+  });
+
+  // Group pointers by index to calculate offsets for overlapping pointers at same position
+  const pointersByIndex = pointers.reduce((acc, pointer) => {
+    const idx = pointer.index;
+    if (!acc[idx]) acc[idx] = [];
+    acc[idx].push(pointer);
+    return acc;
+  }, {} as Record<number, PointerState[]>);
+
+  // Calculate horizontal offset for pointers at the same index
+  const getPointerOffset = (pointer: PointerState): number => {
+    const pointersAtIndex = pointersByIndex[pointer.index] || [];
+    if (pointersAtIndex.length <= 1) return 0;
+
+    const pointerIdx = pointersAtIndex.findIndex(p => p.id === pointer.id);
+    const totalPointers = pointersAtIndex.length;
+    const spacing = 36; // px between overlapping pointers
+    return (pointerIdx - (totalPointers - 1) / 2) * spacing;
+  };
+
   return (
     <div className={cn("flex flex-col items-center", className)}>
       {array.label && (
@@ -58,6 +82,8 @@ export function ArrayView({
               elementWidth={elementWidth}
               gap={gap}
               value={array.elements[pointer.index]?.value}
+              hideValue={hasClosePointers}
+              horizontalOffset={getPointerOffset(pointer)}
             />
           ))}
         </div>

frontend/src/components/visualizations-new/data-structures/linked-list-view.tsx

@@ -0,0 +1,80 @@
+'use client';
+
+import { cn } from '@/lib/utils';
+import type { LinkedListState, LinkedListPointerState } from '@/lib/visualizations/types';
+import { LinkedListNode } from '../primitives/linked-list-node';
+import { LinkedListPointer } from '../primitives/linked-list-pointer';
+
+interface LinkedListViewProps {
+  list: LinkedListState;
+  pointers: LinkedListPointerState[];
+  className?: string;
+}
+
+const NODE_WIDTH = 40; // w-10 = 40px
+const ARROW_WIDTH = 22; // 16px line + 6px arrow head
+
+export function LinkedListView({
+  list,
+  pointers,
+  className,
+}: LinkedListViewProps) {
+  // Calculate total width for proper pointer alignment
+  const totalWidth = list.nodes.length * NODE_WIDTH + (list.nodes.length - 1) * ARROW_WIDTH;
+
+  // Group pointers by nodeIndex to calculate offsets for overlapping pointers
+  const pointersByIndex = pointers.reduce((acc, pointer) => {
+    const idx = pointer.nodeIndex;
+    if (!acc[idx]) acc[idx] = [];
+    acc[idx].push(pointer);
+    return acc;
+  }, {} as Record<number, LinkedListPointerState[]>);
+
+  // Calculate horizontal offset for each pointer
+  const getPointerOffset = (pointer: LinkedListPointerState): number => {
+    const pointersAtIndex = pointersByIndex[pointer.nodeIndex] || [];
+    if (pointersAtIndex.length <= 1) return 0;
+
+    const pointerIdx = pointersAtIndex.findIndex(p => p.id === pointer.id);
+    const totalPointers = pointersAtIndex.length;
+    const spacing = 28; // px between overlapping pointers
+    // Center the group: offset from center based on position
+    return (pointerIdx - (totalPointers - 1) / 2) * spacing;
+  };
+
+  return (
+    <div className={cn('flex flex-col items-center', className)}>
+      {list.label && (
+        <span className="mb-2 text-sm font-medium text-foreground-muted">
+          {list.label}
+        </span>
+      )}
+
+      <div className="relative" style={{ width: totalWidth }}>
+        {/* Pointers row */}
+        <div className="relative mb-1 h-8">
+          {pointers.map((pointer) => (
+            <LinkedListPointer
+              key={pointer.id}
+              pointer={pointer}
+              nodeWidth={NODE_WIDTH}
+              arrowWidth={ARROW_WIDTH}
+              horizontalOffset={getPointerOffset(pointer)}
+            />
+          ))}
+        </div>
+
+        {/* Linked list nodes */}
+        <div className="flex items-center">
+          {list.nodes.map((node, index) => (
+            <LinkedListNode
+              key={node.id}
+              node={node}
+              isLast={index === list.nodes.length - 1}
+            />
+          ))}
+        </div>
+      </div>
+    </div>
+  );
+}

frontend/src/components/visualizations-new/index.ts

@@ -9,10 +9,14 @@ export { VariableInspector } from "./core/variable-inspector";
 export { ArrayElement } from "./primitives/array-element";
 export { Pointer } from "./primitives/pointer";
 export { CalculationBubble } from "./primitives/calculation-bubble";
+export { LinkedListNode } from "./primitives/linked-list-node";
+export { LinkedListPointer } from "./primitives/linked-list-pointer";
 
 // Data structures
 export { ArrayView } from "./data-structures/array-view";
+export { LinkedListView } from "./data-structures/linked-list-view";
 
 // Algorithm visualizations
 export { PrefixSumVisualization } from "./algorithms/prefix-sum";
 export { TwoPointersVisualization } from "./algorithms/two-pointers";
+export { LinkedListVisualization } from "./algorithms/linked-list";

frontend/src/components/visualizations-new/primitives/linked-list-node.tsx

@@ -0,0 +1,64 @@
+'use client';
+
+import { motion } from 'framer-motion';
+import { cn } from '@/lib/utils';
+import type { LinkedListNodeState } from '@/lib/visualizations/types';
+
+interface LinkedListNodeProps {
+  node: LinkedListNodeState;
+  isLast?: boolean;
+  className?: string;
+}
+
+const STATE_CLASSES = {
+  normal: 'bg-surface-variant border-border text-foreground',
+  highlighted: 'bg-primary/20 border-primary text-primary',
+  dimmed: 'bg-surface-variant/50 border-border/50 text-foreground-muted opacity-40',
+  success: 'bg-viz-success/20 border-viz-success text-viz-success',
+  comparing: 'bg-viz-compare/20 border-viz-compare text-viz-compare',
+} as const;
+
+export function LinkedListNode({
+  node,
+  isLast = false,
+  className,
+}: LinkedListNodeProps) {
+  if (node.isNull) {
+    return (
+      <div className={cn('flex items-center', className)}>
+        <div className="flex h-10 items-center justify-center px-3 font-mono text-sm text-foreground-muted">
+          null
+        </div>
+      </div>
+    );
+  }
+
+  return (
+    <div className={cn('flex items-center', className)}>
+      <motion.div
+        layout
+        initial={false}
+        animate={{
+          scale: node.state === 'highlighted' ? 1.05 : 1,
+        }}
+        transition={{ duration: 0.2 }}
+        className={cn(
+          'flex h-10 w-10 items-center justify-center rounded-lg border-2 font-mono font-medium transition-colors duration-200',
+          STATE_CLASSES[node.state]
+        )}
+      >
+        {node.value}
+      </motion.div>
+      {!isLast && (
+        <motion.div
+          initial={{ opacity: 0 }}
+          animate={{ opacity: 1 }}
+          className="flex items-center"
+        >
+          <div className="h-0.5 w-4 bg-border" />
+          <div className="h-0 w-0 border-y-4 border-l-6 border-y-transparent border-l-border" />
+        </motion.div>
+      )}
+    </div>
+  );
+}

frontend/src/components/visualizations-new/primitives/linked-list-pointer.tsx

@@ -0,0 +1,59 @@
+'use client';
+
+import { motion } from 'framer-motion';
+import { cn } from '@/lib/utils';
+import type { LinkedListPointerState } from '@/lib/visualizations/types';
+
+interface LinkedListPointerProps {
+  pointer: LinkedListPointerState;
+  nodeWidth: number;
+  arrowWidth: number;
+  /** Horizontal offset when multiple pointers at same position */
+  horizontalOffset?: number;
+}
+
+const COLOR_CLASSES = {
+  slow: 'text-viz-pointer-left',
+  fast: 'text-viz-pointer-right',
+  prev: 'text-viz-pointer-mid',
+  curr: 'text-viz-pointer-left',
+  next: 'text-viz-pointer-right',
+  default: 'text-viz-pointer-default',
+} as const;
+
+export function LinkedListPointer({
+  pointer,
+  nodeWidth,
+  arrowWidth,
+  horizontalOffset = 0,
+}: LinkedListPointerProps) {
+  // Calculate x position based on node index, with offset for overlapping pointers
+  const x = pointer.nodeIndex * (nodeWidth + arrowWidth) + nodeWidth / 2 + horizontalOffset;
+
+  return (
+    <motion.div
+      initial={false}
+      animate={{ x: x - 12 }}
+      transition={{
+        type: 'spring',
+        stiffness: 300,
+        damping: 30,
+      }}
+      className={cn(
+        'absolute top-0 flex flex-col items-center',
+        COLOR_CLASSES[pointer.color]
+      )}
+    >
+      <span className="text-xs font-medium">{pointer.name}</span>
+      <svg
+        width="16"
+        height="12"
+        viewBox="0 0 16 12"
+        fill="currentColor"
+        className="mt-0.5"
+      >
+        <path d="M8 12L0 0h16L8 12z" />
+      </svg>
+    </motion.div>
+  );
+}

frontend/src/components/visualizations-new/primitives/pointer.tsx

@@ -9,6 +9,10 @@ interface PointerProps {
   elementWidth: number;
   gap: number;
   value?: number;
+  /** Hide the value to save space when pointers are close */
+  hideValue?: boolean;
+  /** Horizontal offset when multiple pointers at same position */
+  horizontalOffset?: number;
   className?: string;
 }
 
@@ -24,10 +28,12 @@ export function Pointer({
   elementWidth,
   gap,
   value,
+  hideValue = false,
+  horizontalOffset = 0,
   className,
 }: PointerProps) {
-  // Calculate center of element: index * (width + gap) + width / 2
-  const offset = pointer.index * (elementWidth + gap) + elementWidth / 2;
+  // Calculate center of element: index * (width + gap) + width / 2, plus any offset for overlapping
+  const offset = pointer.index * (elementWidth + gap) + elementWidth / 2 + horizontalOffset;
 
   return (
     <motion.div
@@ -40,7 +46,7 @@ export function Pointer({
       }}
       className={cn("absolute top-0 h-full", className)}
     >
-      {/* Label - centered above the point */}
+      {/* Label - centered above the arrow */}
       <span
         className={cn(
           "absolute left-0 top-0 -translate-x-1/2 whitespace-nowrap rounded-full px-2 py-0.5 text-xs font-medium",
@@ -48,7 +54,7 @@ export function Pointer({
         )}
       >
         {pointer.name}
-        {pointer.showValue && value !== undefined && (
+        {!hideValue && pointer.showValue && value !== undefined && (
           <span className="ml-1 font-mono">= {value}</span>
         )}
       </span>

frontend/src/content/algorithms/fast-slow-pointers.ts

@@ -0,0 +1,389 @@
+import type { AlgorithmDefinition } from '@/lib/visualizations/types';
+
+export const fastSlowPointersAlgorithm: AlgorithmDefinition = {
+  id: 'fast-slow-pointers',
+  title: 'Find Middle of Linked List',
+  slug: 'fast-slow-pointers',
+  pattern: {
+    name: 'Fast & Slow Pointers',
+    description: 'Use two pointers moving at different speeds to find cycle or middle',
+  },
+  problemStatement:
+    'Given a linked list, find the middle node. If there are two middle nodes, return the second one.',
+  intuition:
+    'If fast pointer moves twice as fast as slow, when fast reaches the end, slow will be at the middle.',
+  code: {
+    language: 'python',
+    code: `def find_middle(head):
+    slow = fast = head
+
+    while fast and fast.next:
+        slow = slow.next
+        fast = fast.next.next
+
+    return slow`,
+  },
+  steps: [
+    // Problem phase
+    {
+      id: 'step-1',
+      phase: 'problem',
+      explanation: 'We have a linked list: 1 → 2 → 3 → 4 → 5 → null. Find the middle node.',
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'normal' },
+              { id: 'n2', value: 2, index: 1, state: 'normal' },
+              { id: 'n3', value: 3, index: 2, state: 'normal' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'n5', value: 5, index: 4, state: 'normal' },
+              { id: 'null', value: 0, index: 5, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [],
+      },
+    },
+    // Intuition phase
+    {
+      id: 'step-2',
+      phase: 'intuition',
+      explanation:
+        'The key insight: if one pointer moves 2x faster than another, when the fast one finishes, the slow one is halfway.',
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'normal' },
+              { id: 'n2', value: 2, index: 1, state: 'normal' },
+              { id: 'n3', value: 3, index: 2, state: 'highlighted' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'n5', value: 5, index: 4, state: 'normal' },
+              { id: 'null', value: 0, index: 5, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [],
+      },
+    },
+    // Code phase
+    {
+      id: 'step-3',
+      phase: 'code',
+      explanation: 'Initialize both pointers at the head of the list.',
+      codeLine: 2,
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'slow', name: 'slow', value: 'head' },
+          { id: 'fast', name: 'fast', value: 'head' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'highlighted' },
+              { id: 'n2', value: 2, index: 1, state: 'normal' },
+              { id: 'n3', value: 3, index: 2, state: 'normal' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'n5', value: 5, index: 4, state: 'normal' },
+              { id: 'null', value: 0, index: 5, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'slow', name: 'slow', nodeIndex: 0, color: 'slow' },
+          { id: 'fast', name: 'fast', nodeIndex: 0, color: 'fast' },
+        ],
+      },
+    },
+    // Execution phase - iteration 1
+    {
+      id: 'step-4',
+      phase: 'execution',
+      explanation: 'Check: fast (node 1) exists and fast.next (node 2) exists. Enter loop.',
+      codeLine: 4,
+      decision: {
+        question: 'Is fast and fast.next valid?',
+        answer: 'Yes, both exist',
+        action: 'Enter the while loop',
+      },
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'slow', name: 'slow', value: 'node 1' },
+          { id: 'fast', name: 'fast', value: 'node 1' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'highlighted' },
+              { id: 'n2', value: 2, index: 1, state: 'normal' },
+              { id: 'n3', value: 3, index: 2, state: 'normal' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'n5', value: 5, index: 4, state: 'normal' },
+              { id: 'null', value: 0, index: 5, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'slow', name: 'slow', nodeIndex: 0, color: 'slow' },
+          { id: 'fast', name: 'fast', nodeIndex: 0, color: 'fast' },
+        ],
+      },
+    },
+    {
+      id: 'step-5',
+      phase: 'execution',
+      explanation: 'Move slow one step forward (1 → 2).',
+      codeLine: 5,
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'slow', name: 'slow', value: 'node 2', previousValue: 'node 1' },
+          { id: 'fast', name: 'fast', value: 'node 1' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'dimmed' },
+              { id: 'n2', value: 2, index: 1, state: 'highlighted' },
+              { id: 'n3', value: 3, index: 2, state: 'normal' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'n5', value: 5, index: 4, state: 'normal' },
+              { id: 'null', value: 0, index: 5, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'slow', name: 'slow', nodeIndex: 1, color: 'slow' },
+          { id: 'fast', name: 'fast', nodeIndex: 0, color: 'fast' },
+        ],
+      },
+    },
+    {
+      id: 'step-6',
+      phase: 'execution',
+      explanation: 'Move fast two steps forward (1 → 2 → 3).',
+      codeLine: 6,
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'slow', name: 'slow', value: 'node 2' },
+          { id: 'fast', name: 'fast', value: 'node 3', previousValue: 'node 1' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'dimmed' },
+              { id: 'n2', value: 2, index: 1, state: 'highlighted' },
+              { id: 'n3', value: 3, index: 2, state: 'comparing' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'n5', value: 5, index: 4, state: 'normal' },
+              { id: 'null', value: 0, index: 5, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'slow', name: 'slow', nodeIndex: 1, color: 'slow' },
+          { id: 'fast', name: 'fast', nodeIndex: 2, color: 'fast' },
+        ],
+      },
+    },
+    // Iteration 2
+    {
+      id: 'step-7',
+      phase: 'execution',
+      explanation: 'Check: fast (node 3) exists and fast.next (node 4) exists. Continue loop.',
+      codeLine: 4,
+      decision: {
+        question: 'Is fast and fast.next valid?',
+        answer: 'Yes, both exist',
+        action: 'Continue the while loop',
+      },
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'slow', name: 'slow', value: 'node 2' },
+          { id: 'fast', name: 'fast', value: 'node 3' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'dimmed' },
+              { id: 'n2', value: 2, index: 1, state: 'highlighted' },
+              { id: 'n3', value: 3, index: 2, state: 'comparing' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'n5', value: 5, index: 4, state: 'normal' },
+              { id: 'null', value: 0, index: 5, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'slow', name: 'slow', nodeIndex: 1, color: 'slow' },
+          { id: 'fast', name: 'fast', nodeIndex: 2, color: 'fast' },
+        ],
+      },
+    },
+    {
+      id: 'step-8',
+      phase: 'execution',
+      explanation: 'Move slow one step forward (2 → 3).',
+      codeLine: 5,
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'slow', name: 'slow', value: 'node 3', previousValue: 'node 2' },
+          { id: 'fast', name: 'fast', value: 'node 3' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'dimmed' },
+              { id: 'n2', value: 2, index: 1, state: 'dimmed' },
+              { id: 'n3', value: 3, index: 2, state: 'highlighted' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'n5', value: 5, index: 4, state: 'normal' },
+              { id: 'null', value: 0, index: 5, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'slow', name: 'slow', nodeIndex: 2, color: 'slow' },
+          { id: 'fast', name: 'fast', nodeIndex: 2, color: 'fast' },
+        ],
+      },
+    },
+    {
+      id: 'step-9',
+      phase: 'execution',
+      explanation: 'Move fast two steps forward (3 → 4 → 5).',
+      codeLine: 6,
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'slow', name: 'slow', value: 'node 3' },
+          { id: 'fast', name: 'fast', value: 'node 5', previousValue: 'node 3' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'dimmed' },
+              { id: 'n2', value: 2, index: 1, state: 'dimmed' },
+              { id: 'n3', value: 3, index: 2, state: 'highlighted' },
+              { id: 'n4', value: 4, index: 3, state: 'dimmed' },
+              { id: 'n5', value: 5, index: 4, state: 'comparing' },
+              { id: 'null', value: 0, index: 5, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'slow', name: 'slow', nodeIndex: 2, color: 'slow' },
+          { id: 'fast', name: 'fast', nodeIndex: 4, color: 'fast' },
+        ],
+      },
+    },
+    // Check and exit
+    {
+      id: 'step-10',
+      phase: 'execution',
+      explanation: 'Check: fast (node 5) exists but fast.next is null. Exit loop.',
+      codeLine: 4,
+      decision: {
+        question: 'Is fast and fast.next valid?',
+        answer: 'No, fast.next is null',
+        action: 'Exit the while loop',
+      },
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'slow', name: 'slow', value: 'node 3' },
+          { id: 'fast', name: 'fast', value: 'node 5' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'dimmed' },
+              { id: 'n2', value: 2, index: 1, state: 'dimmed' },
+              { id: 'n3', value: 3, index: 2, state: 'highlighted' },
+              { id: 'n4', value: 4, index: 3, state: 'dimmed' },
+              { id: 'n5', value: 5, index: 4, state: 'comparing' },
+              { id: 'null', value: 0, index: 5, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'slow', name: 'slow', nodeIndex: 2, color: 'slow' },
+          { id: 'fast', name: 'fast', nodeIndex: 4, color: 'fast' },
+        ],
+      },
+    },
+    // Return result
+    {
+      id: 'step-11',
+      phase: 'execution',
+      explanation: 'Return slow, which points to node 3 — the middle of the linked list!',
+      codeLine: 8,
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'slow', name: 'slow', value: 'node 3' },
+          { id: 'fast', name: 'fast', value: 'node 5' },
+          { id: 'result', name: 'result', value: 3 },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'dimmed' },
+              { id: 'n2', value: 2, index: 1, state: 'dimmed' },
+              { id: 'n3', value: 3, index: 2, state: 'success' },
+              { id: 'n4', value: 4, index: 3, state: 'dimmed' },
+              { id: 'n5', value: 5, index: 4, state: 'dimmed' },
+              { id: 'null', value: 0, index: 5, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'slow', name: 'slow', nodeIndex: 2, color: 'slow' },
+        ],
+      },
+    },
+  ],
+};

frontend/src/content/algorithms/linkedlist-reversal.ts

@@ -0,0 +1,515 @@
+import type { AlgorithmDefinition } from '@/lib/visualizations/types';
+
+export const linkedListReversalAlgorithm: AlgorithmDefinition = {
+  id: 'linkedlist-reversal',
+  title: 'Reverse a Linked List',
+  slug: 'linkedlist-reversal',
+  pattern: {
+    name: 'LinkedList Reversal',
+    description: 'Reverse links between nodes using three pointers',
+  },
+  problemStatement:
+    'Given a singly linked list, reverse it in-place and return the new head.',
+  intuition:
+    'Use three pointers (prev, curr, next) to reverse each link. Save next before breaking the link, then move forward.',
+  code: {
+    language: 'python',
+    code: `def reverse_list(head):
+    prev = None
+    curr = head
+
+    while curr:
+        next_node = curr.next
+        curr.next = prev
+        prev = curr
+        curr = next_node
+
+    return prev`,
+  },
+  steps: [
+    // Problem phase
+    {
+      id: 'step-1',
+      phase: 'problem',
+      explanation: 'We have a linked list: 1 → 2 → 3 → 4 → null. Reverse it to get 4 → 3 → 2 → 1 → null.',
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'normal' },
+              { id: 'n2', value: 2, index: 1, state: 'normal' },
+              { id: 'n3', value: 3, index: 2, state: 'normal' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'null', value: 0, index: 4, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [],
+      },
+    },
+    // Intuition phase
+    {
+      id: 'step-2',
+      phase: 'intuition',
+      explanation:
+        'Key insight: We need to reverse each arrow. To do this safely, save the next node before changing the link.',
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'highlighted' },
+              { id: 'n2', value: 2, index: 1, state: 'comparing' },
+              { id: 'n3', value: 3, index: 2, state: 'normal' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'null', value: 0, index: 4, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [],
+      },
+    },
+    // Code phase - initialization
+    {
+      id: 'step-3',
+      phase: 'code',
+      explanation: 'Initialize prev = None (nothing before head yet) and curr = head.',
+      codeLine: 2,
+      codeHighlightLines: [2, 3],
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'prev', name: 'prev', value: 'None' },
+          { id: 'curr', name: 'curr', value: 'node 1' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'highlighted' },
+              { id: 'n2', value: 2, index: 1, state: 'normal' },
+              { id: 'n3', value: 3, index: 2, state: 'normal' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'null', value: 0, index: 4, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'curr', name: 'curr', nodeIndex: 0, color: 'curr' },
+        ],
+      },
+    },
+    // Execution - iteration 1
+    {
+      id: 'step-4',
+      phase: 'execution',
+      explanation: 'Check: curr (node 1) exists. Enter the while loop.',
+      codeLine: 5,
+      decision: {
+        question: 'Is curr not None?',
+        answer: 'Yes, curr is node 1',
+        action: 'Enter the while loop',
+      },
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'prev', name: 'prev', value: 'None' },
+          { id: 'curr', name: 'curr', value: 'node 1' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'highlighted' },
+              { id: 'n2', value: 2, index: 1, state: 'normal' },
+              { id: 'n3', value: 3, index: 2, state: 'normal' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'null', value: 0, index: 4, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'curr', name: 'curr', nodeIndex: 0, color: 'curr' },
+        ],
+      },
+    },
+    {
+      id: 'step-5',
+      phase: 'execution',
+      explanation: 'Save next_node = curr.next (node 2). This preserves our forward reference.',
+      codeLine: 6,
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'prev', name: 'prev', value: 'None' },
+          { id: 'curr', name: 'curr', value: 'node 1' },
+          { id: 'next', name: 'next', value: 'node 2' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'highlighted' },
+              { id: 'n2', value: 2, index: 1, state: 'comparing' },
+              { id: 'n3', value: 3, index: 2, state: 'normal' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'null', value: 0, index: 4, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'curr', name: 'curr', nodeIndex: 0, color: 'curr' },
+          { id: 'next', name: 'next', nodeIndex: 1, color: 'next' },
+        ],
+      },
+    },
+    {
+      id: 'step-6',
+      phase: 'execution',
+      explanation: 'Reverse the link: curr.next = prev. Node 1 now points to None (backwards!).',
+      codeLine: 7,
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'prev', name: 'prev', value: 'None' },
+          { id: 'curr', name: 'curr', value: 'node 1' },
+          { id: 'next', name: 'next', value: 'node 2' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'success' },
+              { id: 'n2', value: 2, index: 1, state: 'comparing' },
+              { id: 'n3', value: 3, index: 2, state: 'normal' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'null', value: 0, index: 4, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'curr', name: 'curr', nodeIndex: 0, color: 'curr' },
+          { id: 'next', name: 'next', nodeIndex: 1, color: 'next' },
+        ],
+      },
+    },
+    {
+      id: 'step-7',
+      phase: 'execution',
+      explanation: 'Move prev forward: prev = curr. Now prev points to node 1.',
+      codeLine: 8,
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'prev', name: 'prev', value: 'node 1', previousValue: 'None' },
+          { id: 'curr', name: 'curr', value: 'node 1' },
+          { id: 'next', name: 'next', value: 'node 2' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'success' },
+              { id: 'n2', value: 2, index: 1, state: 'comparing' },
+              { id: 'n3', value: 3, index: 2, state: 'normal' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'null', value: 0, index: 4, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'prev', name: 'prev', nodeIndex: 0, color: 'prev' },
+          { id: 'curr', name: 'curr', nodeIndex: 0, color: 'curr' },
+          { id: 'next', name: 'next', nodeIndex: 1, color: 'next' },
+        ],
+      },
+    },
+    {
+      id: 'step-8',
+      phase: 'execution',
+      explanation: 'Move curr forward: curr = next_node. Now curr points to node 2.',
+      codeLine: 9,
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'prev', name: 'prev', value: 'node 1' },
+          { id: 'curr', name: 'curr', value: 'node 2', previousValue: 'node 1' },
+          { id: 'next', name: 'next', value: 'node 2' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'success' },
+              { id: 'n2', value: 2, index: 1, state: 'highlighted' },
+              { id: 'n3', value: 3, index: 2, state: 'normal' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'null', value: 0, index: 4, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'prev', name: 'prev', nodeIndex: 0, color: 'prev' },
+          { id: 'curr', name: 'curr', nodeIndex: 1, color: 'curr' },
+        ],
+      },
+    },
+    // Iteration 2
+    {
+      id: 'step-9',
+      phase: 'execution',
+      explanation: 'Check: curr (node 2) exists. Continue loop.',
+      codeLine: 5,
+      decision: {
+        question: 'Is curr not None?',
+        answer: 'Yes, curr is node 2',
+        action: 'Continue the while loop',
+      },
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'prev', name: 'prev', value: 'node 1' },
+          { id: 'curr', name: 'curr', value: 'node 2' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'success' },
+              { id: 'n2', value: 2, index: 1, state: 'highlighted' },
+              { id: 'n3', value: 3, index: 2, state: 'normal' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'null', value: 0, index: 4, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'prev', name: 'prev', nodeIndex: 0, color: 'prev' },
+          { id: 'curr', name: 'curr', nodeIndex: 1, color: 'curr' },
+        ],
+      },
+    },
+    {
+      id: 'step-10',
+      phase: 'execution',
+      explanation: 'Save next_node = curr.next (node 3).',
+      codeLine: 6,
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'prev', name: 'prev', value: 'node 1' },
+          { id: 'curr', name: 'curr', value: 'node 2' },
+          { id: 'next', name: 'next', value: 'node 3' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'success' },
+              { id: 'n2', value: 2, index: 1, state: 'highlighted' },
+              { id: 'n3', value: 3, index: 2, state: 'comparing' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'null', value: 0, index: 4, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'prev', name: 'prev', nodeIndex: 0, color: 'prev' },
+          { id: 'curr', name: 'curr', nodeIndex: 1, color: 'curr' },
+          { id: 'next', name: 'next', nodeIndex: 2, color: 'next' },
+        ],
+      },
+    },
+    {
+      id: 'step-11',
+      phase: 'execution',
+      explanation: 'Reverse the link: curr.next = prev. Node 2 now points to node 1.',
+      codeLine: 7,
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'prev', name: 'prev', value: 'node 1' },
+          { id: 'curr', name: 'curr', value: 'node 2' },
+          { id: 'next', name: 'next', value: 'node 3' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'success' },
+              { id: 'n2', value: 2, index: 1, state: 'success' },
+              { id: 'n3', value: 3, index: 2, state: 'comparing' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'null', value: 0, index: 4, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'prev', name: 'prev', nodeIndex: 0, color: 'prev' },
+          { id: 'curr', name: 'curr', nodeIndex: 1, color: 'curr' },
+          { id: 'next', name: 'next', nodeIndex: 2, color: 'next' },
+        ],
+      },
+    },
+    {
+      id: 'step-12',
+      phase: 'execution',
+      explanation: 'Move prev = curr, curr = next_node. Advance to process node 3.',
+      codeLine: 8,
+      codeHighlightLines: [8, 9],
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'prev', name: 'prev', value: 'node 2', previousValue: 'node 1' },
+          { id: 'curr', name: 'curr', value: 'node 3', previousValue: 'node 2' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'success' },
+              { id: 'n2', value: 2, index: 1, state: 'success' },
+              { id: 'n3', value: 3, index: 2, state: 'highlighted' },
+              { id: 'n4', value: 4, index: 3, state: 'normal' },
+              { id: 'null', value: 0, index: 4, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'prev', name: 'prev', nodeIndex: 1, color: 'prev' },
+          { id: 'curr', name: 'curr', nodeIndex: 2, color: 'curr' },
+        ],
+      },
+    },
+    // Iteration 3 (condensed)
+    {
+      id: 'step-13',
+      phase: 'execution',
+      explanation: 'Process node 3: save next (node 4), reverse link (3→2), advance pointers.',
+      codeLine: 6,
+      codeHighlightLines: [6, 7, 8, 9],
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'prev', name: 'prev', value: 'node 3', previousValue: 'node 2' },
+          { id: 'curr', name: 'curr', value: 'node 4', previousValue: 'node 3' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'success' },
+              { id: 'n2', value: 2, index: 1, state: 'success' },
+              { id: 'n3', value: 3, index: 2, state: 'success' },
+              { id: 'n4', value: 4, index: 3, state: 'highlighted' },
+              { id: 'null', value: 0, index: 4, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'prev', name: 'prev', nodeIndex: 2, color: 'prev' },
+          { id: 'curr', name: 'curr', nodeIndex: 3, color: 'curr' },
+        ],
+      },
+    },
+    // Iteration 4 (last node)
+    {
+      id: 'step-14',
+      phase: 'execution',
+      explanation: 'Process node 4: save next (null), reverse link (4→3), advance pointers.',
+      codeLine: 6,
+      codeHighlightLines: [6, 7, 8, 9],
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'prev', name: 'prev', value: 'node 4', previousValue: 'node 3' },
+          { id: 'curr', name: 'curr', value: 'None', previousValue: 'node 4' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            nodes: [
+              { id: 'n1', value: 1, index: 0, state: 'success' },
+              { id: 'n2', value: 2, index: 1, state: 'success' },
+              { id: 'n3', value: 3, index: 2, state: 'success' },
+              { id: 'n4', value: 4, index: 3, state: 'success' },
+              { id: 'null', value: 0, index: 4, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [
+          { id: 'prev', name: 'prev', nodeIndex: 3, color: 'prev' },
+        ],
+      },
+    },
+    // Exit and return
+    {
+      id: 'step-15',
+      phase: 'execution',
+      explanation: 'Check: curr is None. Exit loop. Return prev (node 4), the new head!',
+      codeLine: 11,
+      decision: {
+        question: 'Is curr not None?',
+        answer: 'No, curr is None',
+        action: 'Exit loop, return prev',
+      },
+      dataState: {
+        arrays: [],
+        pointers: [],
+        variables: [
+          { id: 'prev', name: 'prev', value: 'node 4' },
+          { id: 'curr', name: 'curr', value: 'None' },
+          { id: 'result', name: 'result', value: 'node 4 (new head)' },
+        ],
+        calculations: [],
+        linkedLists: [
+          {
+            id: 'list',
+            label: 'Reversed: 4 → 3 → 2 → 1 → null',
+            nodes: [
+              { id: 'n4', value: 4, index: 0, state: 'success' },
+              { id: 'n3', value: 3, index: 1, state: 'success' },
+              { id: 'n2', value: 2, index: 2, state: 'success' },
+              { id: 'n1', value: 1, index: 3, state: 'success' },
+              { id: 'null', value: 0, index: 4, state: 'normal', isNull: true },
+            ],
+          },
+        ],
+        linkedListPointers: [],
+      },
+    },
+  ],
+};

frontend/src/lib/visualizations/types.ts

@@ -77,6 +77,11 @@ export interface DataState {
   pointers: PointerState[];
   variables: VariableState[];
   calculations: CalculationState[];
+  // LinkedList support
+  linkedLists?: LinkedListState[];
+  linkedListPointers?: LinkedListPointerState[];
+  // Stack support
+  stacks?: StackState[];
 }
 
 /** Single step in the visualization */
@@ -152,3 +157,49 @@ export interface UseVisualizationReturn {
   currentPhase: VisualizationPhase;
   progress: number;
 }
+
+// ============================================
+// LinkedList Types
+// ============================================
+
+/** State of a linked list node */
+export interface LinkedListNodeState {
+  id: string;
+  value: number;
+  index: number;
+  state: 'normal' | 'highlighted' | 'dimmed' | 'success' | 'comparing';
+  isNull?: boolean;
+}
+
+/** Complete linked list state */
+export interface LinkedListState {
+  id: string;
+  nodes: LinkedListNodeState[];
+  label?: string;
+}
+
+/** Pointer for linked list visualization */
+export interface LinkedListPointerState {
+  id: string;
+  name: string;
+  nodeIndex: number;
+  color: 'slow' | 'fast' | 'prev' | 'curr' | 'next' | 'default';
+}
+
+// ============================================
+// Stack Types
+// ============================================
+
+/** State of a stack element */
+export interface StackElementState {
+  id: string;
+  value: number | string;
+  state: 'normal' | 'highlighted' | 'popped' | 'pushing';
+}
+
+/** Complete stack state */
+export interface StackState {
+  id: string;
+  elements: StackElementState[];
+  label?: string;
+}