MEDIUM NC#56 Blind #68 Trees
98. Validate Binary Search Tree
📖 Problem
Given the root of a binary tree, determine if it is a valid binary search tree (BST). A valid BST is defined as follows: - The left subtree of a node contains only nodes with keys less than the node's key. - The right subtree of a node contains only nodes with keys greater than the node's key. - Both the left and right subtrees must also be binary search trees.
🧠 Visual Learning Aid
1 Model the input into the right structure
2 Choose the core technique and invariant
3 Execute step-by-step with a sample
4 Validate complexity and edge cases
JS/TS Refreshers
- •Array methods (`push`, `pop`, `shift`, `slice`)
- •Object/Map/Set usage patterns
- •Function parameter and return typing
- •In-place array updates
- •Sorted array traversal
- •Boundary condition checks
Logical Thinking Concepts
- •Define invariants before coding
- •Check edge cases first (`[]`, single element, duplicates)
- •Estimate time/space before implementation
- •Apply Two Pointers reasoning pattern
- •Apply Binary Search reasoning pattern
- •Apply DFS reasoning pattern
💡 Approach
- → Use DFS with min/max bounds for each node
- → Initialize bounds as -Infinity and Infinity
- → Left child must be < current node, right child must be > current node
- → Time: O(n), Space: O(h) where h is tree height
🛠️ Hints & Pitfalls
Hints
- •Use DFS with min/max bounds for each node
- •Initialize bounds as -Infinity and Infinity
- •Left child must be < current node, right child must be > current node
Common Pitfalls
- •Time: O(n), Space: O(h) where h is tree height
🧪 Test Cases
Hidden tests on submit: 1
Test Case 1
Not run Input:
validate(node: TreeNode | null, min: number, max: number); Expected:
Computed from hidden reference Test Case 2
Not run Input:
validate(node.left, min, node.val); Expected:
Computed from hidden reference Test Case 3
Not run Input:
validate(node.right, node.val, max); Expected:
Computed from hidden reference 📝 Code Editor
📤 Output