题解 | #二叉树的前序遍历#

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 
# @param root TreeNode类 
# @return int整型
#
class Solution:
    def maxDepth(self , root: TreeNode) -> int:
        if root == None:
            return 0
        
        queue_ = [root]
        result = []
        while queue_:
            length = len(queue_)
            sub = []
            for i in range(length):
                cur = queue_.pop(0)
                sub.append(cur.val)
                #子节点入队列
                if cur.left: queue_.append(cur.left)
                if cur.right: queue_.append(cur.right)
            result.append(sub)
            
        return len(result)