二叉树的最大深度

# 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: TreeNode) -> int:
        result = [1]
        flag = self.Depth(root, result, 1)
        if flag is False:
            return result[0] - 1
        return result[0]
    
    def Depth(self, root, result, level):
        if root is None:
            return False
        if level > result[0]:
            result[0] = level
        self.Depth(root.left, result, level + 1)
        self.Depth(root.right, result, level + 1)
        return True

常规递归法，详情见二叉树通用遍历方法