How do you validate a Binary Search Tree?

Short answer: Each node must lie within an allowed (low, high) range. Recurse left with high = node.val and right with low = node.val. Alternatively, inorder traversal must produce a strictly increasing sequence.

Complexity

Time O(n), Space O(h) recursion height.

Mistakes to avoid

• Only comparing with immediate children (misses deeper violations)
• Using ≤ when duplicates are not allowed

Pass long.MinValue/MaxValue (or nullable bounds) to avoid int overflow edge cases.