Explain Binary Tree Level Order Traversal (BFS).

Short answer: BFS with a queue. Process level size at a time, enqueue children, and collect values per level into a list of lists.

Complexity

Time O(n), Space O(n) for the queue/result.

Common follow-ups

• Zigzag level order
• Average of levels
• Right side view

Capturing level size before the inner loop is the key detail interviewers watch for.