text: from data_structure import TreeNode, build_binary_tree
  
  
  class Solution:
      def postorderTraversal(self, root):
          """
          :type root: TreeNode
          :rtype: List[int]
          """
          if root is None:
              return []
          result = []
          result.extend(self.postorderTraversal(root.left))
          result.extend(self.postorderTraversal(root.right))
          result.append(root.val)
          return result
  
  
  if __name__ == "__main__":
      print(Solution().postorderTraversal(build_binary_tree((1, ((3,), 2)))))
  