[Leetcode][python]Maximum Depth of Binary Tree/二叉树的最大深度

def maxDepth(self, root):
        if root == None:
            return 0
        return max(self.maxDepth(root.left),self.maxDepth(root.right))+1

class Solution(object):
    level_true = 0
    def preorder(self, root, level, level_true):
        if root:
            if level_true < level+1: 
                print(level_true)
                self.level_true += 1
            self.preorder(root.left, level+1, self.level_true)
            self.preorder(root.right, level+1, self.level_true)
    def maxDepth(self, root):
        self.preorder(root, 0, self.level_true)
        return self.level_true