Validate string valid BST
Last updated: October 27, 2025
Quick Overview
Given a binary tree represented as a string, write a function to determine if the tree is a valid binary search tree (BST). The function should return true if the tree satisfies the BST properties, where for every node, all values in the left subtree are less than the node's value, and all values in the right subtree are greater. The input will be a string representation of the tree, and the output should be a boolean value indicating the validity of the BST.
Twitter/X
October 27, 202539
5
2,502 solved
Given a binary tree represented as a string, write a function to determine if the tree is a valid binary search tree (BST). The function should return true if the tree satisfies the BST properties, where for every node, all values in the left subtree are less than the node's value, and all values in the right subtree are greater. The input will be a string representation of the tree, and the output should be a boolean value indicating the validity of the BST.
This coding problem is frequently asked during Onsite at Twitter/X. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Twitter/X 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
- What if the input doesn't fit in memory?
- Can you solve this iteratively instead of recursively (or vice versa)?
- What is the worst-case input for your solution?
- How would you modify your solution to handle streaming input?
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Practice DSA ProblemsSample Answer
Problem Analysis
To determine whether a binary tree represented as a string is a valid binary search tree (BST), we can identify that the problem requires us to validate the properties of a BST. Specifically, for each...
Approach
- Parse the Input: Convert the string representation of the tree into a usable tree structure (e.g., a TreeNode class). For example, if the input string is '3(2)(4)', we should create a tree wher...
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