Python 6 lines BFS solution
\nclass Solution(object):\n    def postorder(self, root):\n        ret, stack = [], root and [root]\n        while stack:\n            node = stack.pop()\n     
```\nclass Solution(object):\n    def postorder(self, root):\n        ret, stack = [], root and [root]\n        while stack:\n            node = stack.pop()\n            ret.append(node.val)\n            stack += [child for child in node.children if child]\n        return ret[::-1]\n```
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