# 104. 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.


# Example 1:
# Input: root = [3,9,20,null,null,15,7]
# Output: 3

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

# Example 3:
# Input: root = []
# Output: 0

# Example 4:
# Input: root = [0]
# Output: 1

# Definition for a binary tree node.
class TreeNode(object):
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right


class Solution(object):
    def maxDepth(self, root):
        if not root:
            return 0
        left = self.maxDepth(root.left)
        right = self.maxDepth(root.right)

        return max(left, right) + 1
