方法一：递归

  1. func postorder(root *Node) []int {
  2.     if root == nil {
  3.         return []int{}
  4.     }
  6.     ret := []int{}
  7.     for i := 0; i < len(root.Children); i++ {
  8.         ret = append(ret, postorder(root.Children[i])...)
  9.     }
  11.     ret = append(ret, root.Val)
  12.     return ret
  13. }