LeetCode:145 二叉树的后序遍历（用栈模拟递归，记录前一个输出的节点）

class Solution {

    public List<Integer> postorderTraversal(TreeNode root) {

       List<Integer> res = new LinkedList<>();

       Deque<TreeNode> stack = new LinkedList<>();

       TreeNode t = root;

       TreeNode prev = null;

       while(t!=null||!stack.isEmpty()){

           if(t!=null){

               stack.push(t);

               t = t.left;

           }

           else{

               t = stack.pop();

               if(t.right==null||t.right==prev){

                   res.add(t.val);

                   prev = t;

                   t = null;

               }

               else{    

                   stack.push(t);

                   t = t.right;

               }

           }

       }

       return res;

    }

}