All problems

Maximum Depth of Binary Tree

Easy · tree dfs

You are given the root of a binary tree. Return its maximum depth: the number of nodes on the longest path that starts at the root and walks down to a leaf.

Count nodes, not edges. A tree with a single node has depth 1. An empty tree has depth 0.

Examples

Example 1

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

Output: 3

The longest path is 3 -> 20 -> 15 (or 3 -> 20 -> 7), which touches 3 nodes.

Example 2

Input: root = [1, null, 2]

Output: 2

A lone right child still counts as a level.

Example 3

Input: root = []

Output: 0

There are no nodes at all.

Constraints