LeetCode 494. Target Sum

class Solution {
public:
    int findTargetSumWays(vector<int>& nums, int S) {
        return cal(0,0,nums,S);
    }
    
    int cal(int index, int sum, vector<int> &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);
    }
};