Optimize traversal for without recursion
Last updated: September 27, 2025
Quick Overview
Given a binary tree, implement an iterative algorithm to traverse the tree in pre-order without using recursion. Your solution should utilize a stack data structure to manage the traversal and return the nodes' values in the order they are visited. The input will be the root node of the binary tree, and the output should be a list of integers representing the pre-order traversal.
OpenAI
September 27, 2025139
12
2,010 solved
Given a binary tree, implement an iterative algorithm to traverse the tree in pre-order without using recursion. Your solution should utilize a stack data structure to manage the traversal and return the nodes' values in the order they are visited. The input will be the root node of the binary tree, and the output should be a list of integers representing the pre-order traversal.
Coding interviews at OpenAI 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
- What if the input doesn't fit in memory?
- Can you solve this iteratively instead of recursively (or vice versa)?
- What happens if the input contains duplicates?
- Can you solve this in a single pass?
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Problem Analysis
To perform a pre-order traversal of a binary tree without recursion, we can utilize an iterative approach with a stack data structure. The pre-order traversal follows the order: visit the root node, t...
Approach
- Initialize an empty stack and an empty result list.
- Push the root node onto the stack.
- While the stack is not empty, do the following:
- Pop the top node from the stack.
- Append it...
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