DFS on graph
Last updated: February 1, 2026
Quick Overview
Implement a Depth-First Search (DFS) algorithm to traverse a given graph represented as an adjacency list. Your function should take in the graph and a starting node, returning a list of nodes in the order they were visited. Ensure that your solution handles cycles and avoids revisiting nodes.
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Implement a Depth-First Search (DFS) algorithm to traverse a given graph represented as an adjacency list. Your function should take in the graph and a starting node, returning a list of nodes in the order they were visited. Ensure that your solution handles cycles and avoids revisiting nodes.
This coding problem is frequently asked during Phone Screen at Reddit. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Reddit expects candidates to write production-quality code, not just solve the puzzle.
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 your solution change if the input was sorted?
- How would you parallelize this solution?
- What if the input doesn't fit in memory?
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Practice DSA ProblemsSample Answer
Problem Analysis
The problem requires us to traverse a graph using Depth-First Search (DFS) algorithm. The graph is represented as an adjacency list, which indicates the neighbors of each node. A DFS is a suitable cho...
Approach
To implement DFS, we can use a recursive function or an explicit stack. Here’s the step-by-step algorithm:
- Initialize a set to keep track of visited nodes.
- Define a recursive function t...
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