def test_1_null_2_3_returns_3_2_1(self):
btree = list(TreeNode(i) for i in range(1, 4))
btree[0].right = btree[1]
btree[1].left = btree[2]
self.assertEqual(Solution().postorder_traversal(btree[0]), [3, 2, 1])
def test_1_null_2_returns_2_1(self):
btree = [TreeNode(1), TreeNode(2)]
btree[0].right = btree[1]
self.assertEqual(Solution().postorder_traversal(btree[0]), [2, 1])
def test_1_returns_1(self):
self.assertEqual(Solution().postorder_traversal(TreeNode(1)), [1])
