Binary Tree Postorder Traversal

public class Solution {
    public ArrayList<Integer> postorderTraversal(TreeNode root) {
        ArrayList<Integer> res = new ArrayList<Integer>();
        if(root==null) return res;
        Stack<TreeNode> st = new Stack<TreeNode>();
        Stack<TreeNode> out= new Stack<TreeNode>();
        st.push(root);
        while(!st.isEmpty()){
            TreeNode n= st.pop();
            out.push(n);
            if(n.left!=null){
                st.push(n.left);
            }
            if(n.right!=null){
                st.push(n.right);
            }
        }
        while(!out.isEmpty()){
            TreeNode o = out.pop();
            res.add(o.val);
        }
        return res;
    }
}