Lesson 101
Tree BFS — Level Order Traversal
Breadth-First Search with a Queue
1:00How to traverse a binary tree level by level using a FIFO queue — the breadth-first pattern that also underlies shortest-path algorithms.
By the end, you can
- Explain why a queue produces breadth-first (level-order) output while a stack produces depth-first output.
- Trace BFS step by step on a small tree, showing the queue state after each enqueue and dequeue.
- Implement level-order traversal in Python using `collections.deque`, including the `levelSize` snapshot and null-checks on children.
- State the time complexity (O(n)) and space complexity (O(w)) of BFS and justify both.
- Identify and fix each of the four common BFS pitfalls.
- Recognise which tree and graph problems are naturally solved with a level-order / BFS approach.
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