Validate string valid BST
Last updated: January 14, 2026
Quick Overview
Given a binary tree, implement a function to validate whether it is a valid binary search tree (BST). A valid BST is defined as a tree where for each node, all values in its left subtree are less than the node's value, and all values in its right subtree are greater than the node's value. The function should return a boolean indicating whether the tree satisfies the BST property.
Grubhub
January 14, 202671
11
4,212 solved
Given a binary tree, implement a function to validate whether it is a valid binary search tree (BST). A valid BST is defined as a tree where for each node, all values in its left subtree are less than the node's value, and all values in its right subtree are greater than the node's value. The function should return a boolean indicating whether the tree satisfies the BST property.
This coding problem is frequently asked during Phone Screen at Grubhub. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Grubhub expects candidates to write production-quality code, not just solve the puzzle.
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
- Can you solve this iteratively instead of recursively (or vice versa)?
- What if the input doesn't fit in memory?
- What happens if the input contains duplicates?
- 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 need to ensure that for every node in the tree, all values in its left subtree are less than the node's value and all values in it...
Approach
- Define a recursive function
is_valid_bst(node, low, high)that checks if the current node's value is within the range (low,high). - If the node is
None, returnTrue(an empty tree is ...
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