Maximum Depth of Binary Tree

Easy

BFS

Given the root of a binary tree, return its maximum depth.

A binary tree's maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

While this can be solved with DFS, the BFS approach counts the number of levels traversed, which directly gives us the depth.

Example 1

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

Output: 3

Explanation: The tree has 3 levels: root (3), children (9, 20), grandchildren (15, 7).

Example 2

Input: root = [1,null,2]

Output: 2

Explanation: The tree has 2 levels: root (1) and its right child (2).

Constraints

•The number of nodes in the tree is in the range [0, 10^4]
•-100 <= Node.val <= 100

Show Hints (4)

Hint 1: BFS naturally processes nodes level by level.

Hint 2: Count how many levels you traverse before the queue becomes empty.

Hint 3: Each iteration of the outer while loop processes exactly one level.

Hint 4: The depth equals the number of levels traversed.

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