//return each level sum;
public int sum; 
    public int levelSum(TreeNode root, int level) {
         helper(root, 1, level);
         return sum;
    }
    public void helper(TreeNode root, int depth, int level){
        if (root == null) {
            return;
        }
        if (depth == level) {
            sum += root.val;
        }
        helper(root.left, depth + 1, level);
        helper(root.right, depth + 1, level);
    }