Validate graph valid BST
Last updated: January 10, 2026
Quick Overview
Given a binary tree, implement a function to determine if 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. The function should return a boolean indicating whether the tree satisfies the BST properties.
Doordash
January 10, 20267
4
2,177 solved
Given a binary tree, implement a function to determine if 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. The function should return a boolean indicating whether the tree satisfies the BST properties.
Coding interviews at Doordash 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
- What happens if the input contains duplicates?
- Can you optimize the space complexity of your solution?
- What if the input doesn't fit in memory?
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Problem Analysis
To determine if a binary tree is a valid binary search tree (BST), we can employ a depth-first search (DFS) approach. The key property of a BST is that for any given node, all values in its left subtr...
Approach
- Initialize a helper function
is_valid_bst(node, lower, upper)that takes a node and its valid value bounds (lower and upper). - If the node is
None, returnTrue(an empty tree is a valid B...
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