108th LeetCode Weekly Contest Binary Subarrays With Sum

In an array A of 0s and 1s, how many non-empty subarrays have sum S?

Example 1:

Input: A = [1,0,1,0,1], S = 2
Output: 4
Explanation: 
The 4 subarrays are bolded below:
[1,0,1,0,1]
[1,0,1,0,1]
[1,0,1,0,1]
[1,0,1,0,1]

Note:

1. A.length <= 30000
2. 0 <= S <= A.length
3. A[i] is either 0 or 1.

通用解法就是求连续数组的和有多少个，这种题代码都不会变的

把和存起来，给后面的数字-S看有没有这个和，有的话加起来，然后a[ans]++说明又存在符合条件的解了

class Solution {
public:
    int numSubarraysWithSum(vector<int>& A, int S) {
       long long ans = 0;
       long long k=0;
       map<long long,long long>a;
       a[0]=1;
       int len = A.size();
       for(int i=0;i<len;i++){
        ans+=A[i];
        if(ans>=S){
            k+=a[ans-S];
        }
        a[ans]++;
       }
       return k;
    }
};