Path Sum II

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