剑指32 从上到下打印二叉树

二叉树的层序遍历，本质就是广度遍历有向图。都要用到队列。

# -*- coding:utf-8 -*-
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    # 返回从上到下每个节点值列表，例：[1,2,3]
    def PrintFromTopToBottom(self, root):
        # write code here
        deque = []
        if not root:
            return []
        new = [root]
        while len(new):
            temp = new.pop(0)
            deque.append(temp.val)
            if temp.left:
                new.append(temp.left)
            if temp.right: 
                new.append(temp.right)
        return deque