Postorder Travesal without Recursion

PostorderIterative.java

public ArrayList<Integer> postorderTraversal(TreeNode A) {
        
        Stack<TreeNode> s1 = new Stack<>();
        Stack<TreeNode> s2 = new Stack<>();
        
        if(A == null){
            return null;
        }
        
        s1.push(A);
        
        while(!s1.isEmpty()){
            TreeNode temp = s1.pop();
            s2.push(temp);
            if(temp.left != null){
                s1.push(temp.left);
            }
            if(temp.right != null){
                s1.push(temp.right);
            }
        }
        
        ArrayList<Integer> result = new ArrayList<>();
        while(!s2.isEmpty()){
            result.add(s2.pop().val);
        }
        return result;
    }