Binary Tree Level Order Traversal

Given the root of a binary tree, return the level order traversal of its nodes' values (i.e., from left to right, level by level).

LeetCode: Binary Tree Level Order Traversal

Example:

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]

    3
   / \
  9  20
    /  \
   15   7

Level 0: [3]
Level 1: [9, 20]
Level 2: [15, 7]

Hints

Hint 1

Use a queue to process nodes level by level. This is a classic BFS pattern.

Hint 2

The key trick: capture the queue size at the start of each level, then process exactly that many nodes.

Hint 3

If you don't capture the size, the queue grows during the loop and you'll mix levels together.