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二叉树的层次遍历

描述

给你一个二叉树，请你返回其按 层次遍历 得到的节点值（即逐层地，从做到右访问所有节点）

代码

通过两个数组来交替打印

class Solution(object):def levelOrder(self, root):if root == None:return []stack = [root]secondStack = []ret = [[root.val]]while stack or secondStack:tempList = []while stack:                node = stack.pop(0)                if node.left:secondStack.append(node.left)tempList.append(node.left.val)if node.right:secondStack.append(node.right)tempList.append(node.right.val)if tempList != []:ret.append(tempList)tempList = []while secondStack:node = secondStack.pop(0)if node.left:stack.append(node.left)tempList.append(node.left.val)if node.right:stack.append(node.right)tempList.append(node.right.val)if tempList != []:ret.append(tempList)return ret