What common BFS mistake mixes levels?

Question

What common BFS mistake mixes levels?

Option A — Correct answer

Using changing queue size inside the level loop

Option B

Taking the queue size before the loop

Option C

Adding children after popping a node

Option D

Returning grouped levels

Explanation

Using the changing queue size inside the loop can process next-level nodes too early.

Binary Tree Level Order Traversal - BFS queue Pattern Practice Quiz