Maximum Depth of Binary Tree

Maximum Depth of Binary Tree

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.

Examples

Example 1:

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

      3
     / \
    9  20
       / \
      15   7

Example 2:

Input: root = [1, null, 2]
Output: 2

Example 3:

Input: root = []
Output: 0

Constraints

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

Expected Complexity

• Time: O(n)
• Space: O(h) where h is the height of the tree