feat(content): function signatures + test cases

backend/data/questions/balanced-binary-tree.yaml

@@ -10,6 +10,28 @@ patterns:
   - dfs
   - tree-traversal
 
+function_signature: "def is_balanced(root: TreeNode) -> bool:"
+
+test_cases:
+  visible:
+    - input: { root: [3, 9, 20, null, null, 15, 7] }
+      expected: true
+    - input: { root: [1, 2, 2, 3, 3, null, null, 4, 4] }
+      expected: false
+    - input: { root: [] }
+      expected: true
+  hidden:
+    - input: { root: [1] }
+      expected: true
+    - input: { root: [1, 2, null, 3] }
+      expected: false
+    - input: { root: [1, null, 2, null, 3] }
+      expected: false
+    - input: { root: [1, 2, 3, 4, 5, 6, 7] }
+      expected: true
+    - input: { root: [1, 2, 2, 3, null, null, 3, 4, null, null, 4] }
+      expected: false
+
 description: |
   Given a binary tree, determine if it is **height-balanced**.