930. Binary Subarrays With Sum

class Solution {
public:
    int numSubarraysWithSum(vector<int>& A, int S) {
    int n = A.size();
        if(S==0) return helper(A, n);
        int lo=0, hi=0, prev =-1, count=0, sum =0;
        while(hi<n && A[hi]!=1)
            lo++, hi++;
        
        while(hi<n)
        {
            if(sum +A[hi]==S)
            {
                count+=lo -prev;
                sum+=A[hi];
                hi++;
            }
            else
                if(sum+A[hi]>S)
                {
                    sum-=A[lo];
                    prev =lo;
                    lo++;
                    while(A[lo]!=1)lo++;
                }
            else
            {
                sum += A[hi];
                hi++;
            }
        }
        return count;
            
    }
    int helper(vector<int> A,int n)
    {
        int count =0, ans=0;
        for(int i=0;i<n;i++)
        {
            if(A[i]==0)
                count++, ans+=count;
            else
                count=0;
        }
        return ans;
    }
};