Binary Tree Postorder Traversal

public class Solution {
    public ArrayList<Integer> postorderTraversal(TreeNode root) {
        ArrayList<Integer> res = new ArrayList<Integer>();
        if(root==null) return res;
        TreeNode cur = null, pre = null;
        Stack<TreeNode>stack = new Stack<TreeNode>();
        stack.push(root);
        while(!stack.isEmpty()){
            cur = stack.peek();
            if((cur.right==null && cur.left==null) ||(pre!=null && (cur.right==pre || cur.left==pre))){
                res.add(cur.val);
                pre = cur;
                stack.pop();
            }else{
                if(cur.right!=null){
                    stack.push(cur.right);
                }
                if(cur.left!=null){
                    stack.push(cur.left);
                }
            }
        }
        return res;
    }
}