二叉树的层序遍历，利用队列先进先出的性质

# 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
        if not root:return ''
        queue = [root]
        res = []
        while queue:
            if queue[0].left:
                queue.append(queue[0].left)
            if queue[0].right:
                queue.append(queue[0].right)
            res.append(queue[0].val)
            queue.pop(0)
        return res