非遞歸的方式後序遍歷二叉樹

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.*;
public class Solution {
    public ArrayList<Integer> postorderTraversal(TreeNode root) {
        ArrayList<Integer> als = new ArrayList<Integer>();

        if(root == null){
            return als;
        }
        Stack<TreeNode> s1 = new Stack<TreeNode>();
        Stack<TreeNode> s2 = new Stack<TreeNode>();
        TreeNode iter = root;
        s1.push(iter);
        while(!s1.isEmpty()){
            TreeNode head = s1.pop();
            s2.push(head);
            if(head.left != null){
                s1.push(head.left);
            }
            if(head.right != null){
                s1.push(head.right);
            }
        }
        while(!s2.isEmpty()){
            als.add(s2.pop().val);
        }
        return als;
    }
}