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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 []
stack = [root]
ret = []
while stack:
tmp = stack[0]
ret.append(tmp.val)
if tmp.left:
stack.append(tmp.left)
if tmp.right:
stack.append(tmp.right)
del stack[0]
return ret
