Python 從上往下打印二叉樹

從上往下打印出二叉樹的每個節點，同層節點從左至右打印。

class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution:
    def PrintFromTopToBottom(self,root):
        if root == None:
            return []
        support = [root]
        ret =[]
        while support:
            tmpNode = support[0]
            ret.append(tmpNode.val)
            if tmpNode.left:
                support.append(tmpNode.left)
            if tmpNode.right:
                support.append(tmpNode.right)
            del support[0]
        return  ret