剑指 从上到下打印二叉树

层序遍历 树

import collections
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
        queue=collections.deque([root])
        visit=[]
        result=[]
        if root==None:
            return result
        while queue:
            root=queue.popleft()
            if root.left:
                queue.append(root.left)
            if root.right:
                queue.append(root.right)
            if root not in visit:
                result.append(root.val)
                visit.append(root)
        return result