Validate graph valid BST
Last updated: December 24, 2025
Quick Overview
Given a binary tree, implement a function to determine if it is a valid binary search tree (BST). A valid BST must satisfy the property that for any given node, all values in its left subtree are less than the node's value, and all values in its right subtree are greater. The function should return a boolean indicating whether the tree is a valid BST.
CrowdStrike
December 24, 202522
13
267 solved
Given a binary tree, implement a function to determine if it is a valid binary search tree (BST). A valid BST must satisfy the property that for any given node, all values in its left subtree are less than the node's value, and all values in its right subtree are greater. The function should return a boolean indicating whether the tree is a valid BST.
Coding interviews at CrowdStrike focus on problem-solving approach as much as the final solution. The interviewer wants to see you break down the problem, consider edge cases, and optimize iteratively. Communication throughout the process is key.
What the Interviewer Expects
- Recognize the underlying problem pattern (sliding window, two pointers, BFS/DFS, etc.)
- Discuss multiple approaches and trade-offs before coding
- Implement an optimal solution with clean, production-quality code
- Handle all edge cases including boundary conditions and invalid input
- Optimize both time and space complexity with clear justification
- Test your solution systematically with well-chosen examples
Key Topics to Cover
How to Approach This
- Clarify input constraints and edge cases before writing code.
- Walk through your approach verbally and confirm with the interviewer before coding.
- Start with a brute force solution, then optimize. Mention time and space complexity.
- Test your solution with examples, including edge cases like empty input or duplicates.
- Consider common patterns: sliding window, two pointers, hash map, BFS/DFS, dynamic programming.
Possible Follow-up Questions
- How would you modify your solution to handle streaming input?
- Can you solve this iteratively instead of recursively (or vice versa)?
- Can you optimize the space complexity of your solution?
- Can you solve this in a single pass?
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Problem Analysis
To determine if a binary tree is a valid binary search tree (BST), we must confirm that for every node, all values in its left subtree are less than the node's value, and all values in its right subtr...
Approach
- Define a recursive helper function that takes a node and two bounds (min and max) to check if the current node's value falls within these bounds.
- For each node:
- If it is null, return True (...
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