Q104 Maximum Depth of Binary Tree

Given binary tree [3,9,20,null,null,15,7],
    3
   / \
  9  20
    /  \
   15   7
return its depth = 3.

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

class Solution:
    def maxDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if root == None:
            return 0
        return max(Solution.maxDepth(self, root.left), Solution.maxDepth(self, root.right)) + 1  # 每递归一次，深度加1