Leetcode-Binary Tree Postorder Traversal

Given a binary tree, return the postorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    
     2
    /
   3

return [3,2,1].

Note: Recursive solution is trivial, could you do it iteratively?

Solution:

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        if (root==null) return res;

        Stack<TreeNode> nStack = new Stack<TreeNode>();
        Stack<Integer> record = new Stack<Integer>();
        nStack.push(root);
        record.push(0);

        while (nStack.size()!=0){
            TreeNode cur = nStack.peek();
            if (cur.left==null && cur.right==null){
                res.add(cur.val);
                nStack.pop();
                record.pop();
                continue;
            }

            int val = record.peek();
            if (val==0){
                record.pop();
                record.push(val+1);
                if (cur.left!=null){
                    nStack.push(cur.left);
                    record.push(0);
                }
                continue;
            }

            if (val==1){
                record.pop();
                record.push(val+1);
                if (cur.right!=null){
                    nStack.push(cur.right);
                    record.push(0);
                }
                continue;
            }

            if (val==2){
                res.add(cur.val);
                nStack.pop();
                record.pop();
                continue;
            }
        }

        return res;  
        
    }
}