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Concept

Overview

Process one whole level per iteration
step 1 / 16
binary tree
4
2
5
1
6
3
7
queue (FIFO)
front →
(empty)
← back
Concept
▸
1bfs_by_level(root):
2    queue = [root]
3    while queue not empty:
4        levelSize = len(queue)   // freeze: one level
5        for _ in range(levelSize):
6            node = queue.dequeue()
7            enqueue node.children
8        // whole level in hand — sum / right-view / zigzag / width
9    // advance to next level
10    // done
state
  • ideaone level per loop

line 1Plain BFS visits one node at a time. But almost every interesting tree question — level sums, the right-side view, zigzag order, the widest level — needs to know WHICH LEVEL each node sits on. The trick that unlocks all of them is to process the queue ONE WHOLE LEVEL per outer iteration.