/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        List<Integer> path = new ArrayList<Integer>();
        helper(root, sum, path, res);
        return res;
    }
    
    private void helper (TreeNode root, int sum, List<Integer> path, List<List<Integer>> res) {
        if (root == null) {
            
            return;
        }

        path.add(root.val);
        if (root.left == null && root.right == null) {
            if (sum == root.val) {
                res.add(new ArrayList<Integer>(path));
            }
            path.remove(path.size() - 1);
            return;
        }
        helper(root.left, sum - root.val, path, res);
        helper(root.right, sum - root.val, path, res);
        path.remove(path.size() - 1);
        return;
    }
}