一直觉得二叉树的后续遍历非常绕，所以这里就结合自己的感受来阐述一下我的理解
微信图片_20191224113052.jpg
def lastOrder(root):
if not root:
return None
tmp = root
stack = []
while tmp or stack:
while tmp:
stack.append(tmp)
tmp = tmp.left
node = stack[-1]
tmp = node.right
if tmp is None:
node = stack.pop()
print(node.val)
while stack and node == stack[-1].right:
node = stack.pop()
print(node.val)
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