public int findTargetSumWays(int[] nums, int S) {
findTargetSum(0, 0, S, nums, res);
public void findTargetSum(int index, int curSum, int target, int[] nums, int[] res) {
if (index == nums.length) {
findTargetSum(index + 1, curSum + nums[index], target, nums, res);
findTargetSum(index + 1, curSum - nums[index], target, nums, res);