Simple use post order traversal with an additional variable order. And when visiting a node, add the node's value to the order list.
"""
# Definition for a Node.
class Node:
def __init__(self, val=None, children=None):
self.val = val
self.children = children
"""
class Solution:
def postorder(self, root: 'Node') -> List[int]:
order = []
self._helper(root, order)
return order
def _helper(self, node, order):
if node is None:
return
for child in node.children:
self._helper(child, order)
order.append(node.val)