Validate linked list valid BST
Last updated: January 14, 2026
Quick Overview
Given a linked list where each node contains a value, determine if the values in the linked list can represent 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. Your function should return a boolean indicating whether the linked list can form a valid BST when the values are inserted in the order they appear.
Doordash
January 14, 20265
15
619 solved
Given a linked list where each node contains a value, determine if the values in the linked list can represent 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. Your function should return a boolean indicating whether the linked list can form a valid BST when the values are inserted in the order they appear.
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
- Quickly identify the optimal approach and its theoretical basis
- Handle complex algorithm design with multiple interacting components
- Write concise, elegant code under time pressure
- Prove correctness of your approach and discuss alternative solutions
- Optimize beyond the obvious: discuss constant factor improvements
- Address follow-up variations and explain how the solution generalizes
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 optimize the space complexity of your solution?
- What is the worst-case input for your solution?
- How would your solution change if the input was sorted?
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Problem Analysis
To determine if the values in a linked list can represent a valid binary search tree (BST), we need to ensure that for any given node, all values in its left subtree are less than the node's value, an...
Approach
- Extract Values: Traverse the linked list and extract values into an array. This allows us to easily access the values in order.
- Recursive Validation: Implement a recursive function tha...
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