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Validate Binary Search Tree – Solution & Complexity

Interview solutions
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Validate Binary Search Tree

Solution Walkthrough

  1. Step 1: Recognize the Pattern

    The key pattern is: DFS carries the full valid value range. Identify the state and invariant before coding.

  2. Step 2: Build the Algorithm

    Advance one state transition at a time. Mark or update state before exploring dependent work.

  3. Step 3: Check Edge Cases

    Test empty or minimal input, skewed shapes, duplicates where allowed, and impossible outcomes.

  4. Step 4: Solution and Complexity

    Time: O(n). Space: O(h).

    Java/Go/Rust below are reference solutions for learning the pattern in those languages; only Python and JavaScript can be run and submitted for this problem, since the remote judge doesn't support the TreeNode structure yet.

    def is_valid_bst(root: TreeNode) -> bool:
    def valid(node, low, high):
        if node is None:
            return True
        return low < node.val < high and valid(node.left, low, node.val) and valid(node.right, node.val, high)
    return valid(root, float("-inf"), float("inf"))
Learn the Tree Traversal pattern: recognition cues, invariant, and ordered practice →

FAQ

See also

Practice Validate Binary Search Tree online (run code in the browser)Kth Smallest Element in a BST solution (medium)Lowest Common Ancestor of a BST solution (medium)Balanced Binary Tree solution (easy)Binary Tree Inorder Traversal solution (easy)All coding interview problems
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