class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {

        Stack<TreeNode> s1 = new Stack<>();
        Stack<TreeNode> s2 = new Stack<>();
        s1.add(root);
        List<List<Integer>> res = new ArrayList<>();
        if(root == null)
            return res;

        List<Integer> level = new ArrayList<>();
        while(!s1.isEmpty() || !s2.isEmpty()){
            level = new ArrayList<>();
            while(!s1.isEmpty()){
                TreeNode temp = s1.pop();
                level.add(temp.val);
                if(temp.left != null)
                    s2.push(temp.left);
                if(temp.right != null)
                    s2.push(temp.right);
            }
            res.add(level);

            // reversing the elemenst in stack s1 by putting them in s2
            while(!s2.empty())
            {
                TreeNode temp = s2.pop();
                s1.push(temp);
            }
        }

        return res;
    }
}