剑指 Offer 34. 二叉树中和为某一值的路径

List<List<Integer>> res = new ArrayList<>();
    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        List<Integer> list = new ArrayList<>();
        trackback(root,sum,list);
        return res;
    }

    private void trackback(TreeNode root, int sum, List<Integer> list) {
        if(root == null) return;
        if(root.val==sum && root.left==null && root.right == null) {
            list.add(root.val);
            res.add(new ArrayList<>(list));
            list.remove(list.size()-1);
            return;
        }
        list.add(root.val);
        trackback(root.left,sum - root.val,list);
        trackback(root.right,sum-root.val,list);
        list.remove(list.size()-1);
    }