List<List<Integer>> ans = new ArrayList<>();
    public List<List<Integer>> findPath(TreeNode root, int sum) {
        if(root == null || sum == 0) return ans;
        help(root, sum , new ArrayList<Integer>());
        return ans;
    }

    //回溯
    private void help(TreeNode node, int target, List<Integer> list) {
        if(node == null) return;//终止条件
        //选择
        list.add(node.val);
        target -= node.val;
        if(target == 0 && node.left == null && node.right == null) {
            //找到一个结果
            ans.add(new ArrayList<>(list));
        }else {
            //drill down下一层
            help(node.left, target, list);
            help(node.right, target, list);
        }
        //撤销
        list.remove(list.size() - 1);
    }