104. Maximum Depth of Binary Tree

class Solution(object):
    def maxDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if not root:
            return 0

        left = self.maxDepth(root.left)
        right = self.maxDepth(root.right)
        res = max(left, right) + 1
        return res