Binary Tree Path Sum Lintcode

public class Solution {
    /**
     * @param root the root of binary tree
     * @param target an integer
     * @return all valid paths
     */
    public List<List<Integer>> binaryTreePathSum(TreeNode root, int target) {
        List<List<Integer>> res = new ArrayList<>();
        
        helper(root, target, res, new ArrayList<Integer>());
        return res;
    }
    public TreeNode helper(TreeNode root, int target, List<List<Integer>> res, List<Integer> sub) {
        if (root == null) {
            return null;
        }
        sub.add(root.val);
        
        TreeNode left = helper(root.left, target - root.val, res, new ArrayList<>(sub));
        TreeNode right = helper(root.right, target - root.val, res, new ArrayList<>(sub));
        
        if (left == null && right == null && target - root.val == 0) {
            res.add(sub);
        }
        
        return root;
        
    }
}