feat(content): more test cases

backend/data/questions/validate-binary-search-tree.yaml

@@ -10,6 +10,24 @@ patterns:
   - dfs
   - tree-traversal
 
+function_signature: "def is_valid_bst(root: TreeNode | None) -> bool:"
+
+test_cases:
+  visible:
+    - input: { root: [2, 1, 3] }
+      expected: true
+    - input: { root: [5, 1, 4, null, null, 3, 6] }
+      expected: false
+    - input: { root: [1] }
+      expected: true
+  hidden:
+    - input: { root: [5, 4, 6, null, null, 3, 7] }
+      expected: false
+    - input: { root: [2, 2, 2] }
+      expected: false
+    - input: { root: [10, 5, 15, null, null, 6, 20] }
+      expected: false
+
 description: |
   Given the `root` of a binary tree, determine if it is a valid **binary search tree (BST)**.