104. Maximum Depth of Binary Tree

Easy

---

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.

Example 1:

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

Example 2:

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

Constraints:

• The number of nodes in the tree is in the range [0, 104].

• -100 <= Node.val <= 100

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def maxDepth(self, root: Optional[TreeNode]) -> int:
        result = 0
        def recursion(root, depth):
            nonlocal result
            if root is None:
                if depth > result:
                    result = depth
                return
            recursion(root.left, depth+1)
            recursion(root.right, depth+1)
        recursion(root, 0)
        return result