When do you use BFS versus DFS on a graph?
PICTURE THIS: DATA SPLIT
Fit on train, tune on val, report on test once.
Simple meaning
BFS explores level by level and finds shortest paths in unweighted graphs, using a queue.
WHY — Graphs instead of guessing?
Why interviewers care about Graphs:
contrast on Graphs, not two memorised paragraphs.
the developer, then one case where picking wrong hurts.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1BFS explores level by
level and finds shortest paths in unweighted graphs, using a queue.
- 2DFS goes deep first,
uses a stack or recursion, and fits cycle detection, topological ideas, and path existence.
- 3How it works
Both are O(V+E) time
- 4BFS space can be
the frontier width, DFS space is the recursion depth.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
BFS explores level by level and finds shortest paths in unweighted graphs, using a queue. DFS goes deep first, uses a stack or recursion, and fits cycle detection, topological ideas, and path existence.