给定一个二叉树，返回它的 后序 遍历。

题目：给定一个二叉树，返回它的 后序 遍历。
解法：迭代
思路：利用栈实现中右左，再翻转成左右中
代码：
/**

• Definition for a binary tree node.
• public class TreeNode {
• int val;
• TreeNode left;
• TreeNode right;
• TreeNode() {}
• TreeNode(int val)
• TreeNode(int val, TreeNode left, TreeNode right) {
• this.val = val;
• this.left = left;
• this.right = right;
• }
• }
  */
  class Solution {
  public List postorderTraversal(TreeNode root) {
  LinkedList res = new LinkedList();
  Stack stack = new Stack();
  if(root==null){
  return res;
  }
  stack.push(root);
  while(!stack.isEmpty()){
  TreeNode popNode = stack.pop();
  res.addFirst(popNode.val);
  if(popNode.left!=null){
  stack.push(popNode.left);
  }
  if(popNode.right!=null){
  stack.push(popNode.right);
  }
  }
  return res;
  }
  }