Binary Tree Level Order Traversal

Easy

BFS

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).

A binary tree level order traversal visits all nodes at depth d before visiting nodes at depth d+1.

This is the most fundamental BFS problem - it directly implements the core concept of exploring a tree level by level using a queue.

Example 1

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

Output: [[3],[9,20],[15,7]]

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

Example 2

Input: root = [1]

Output: [[1]]

Explanation: Single node tree has only one level.

Example 3

Input: root = []

Output: []

Explanation: Empty tree returns empty result.

Constraints

•The number of nodes in the tree is in the range [0, 2000]
•-1000 <= Node.val <= 1000

Show Hints (4)

Hint 1: What data structure allows us to process nodes in the order they were discovered?

Hint 2: A queue processes elements in FIFO order - first discovered, first processed.

Hint 3: To separate levels, track how many nodes are in the current level before processing.

Hint 4: For each level, process exactly the number of nodes that were in the queue when the level started.

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