class Solution {public:    int findTargetSumWays(vector
     
      & nums, int S) {        return cal(0,0,nums,S);    }        int cal(int index, int sum, vector
      
        &nums, int S){        if (index==nums.size()){            if (sum==S) return 1;            else return 0;        }        return cal(index+1,sum+nums[index],nums,S)+cal(index+1,sum-nums[index],nums,S);    }};