while(begin < n) { while(end < n && !s.count(prefix[end + 1])) { ++end; s.insert(prefix[end]); } ans += end - begin; s.erase(prefix[begin]); ++begin; } code example

Example: build a prefix array cpp

void fillPrefixSum(int arr[], int n, int prefixSum[]) 
{ 
    prefixSum[0] = arr[0]; 
  
    // Adding present element 
    // with previous element 
    for (int i = 1; i < n; i++) 
        prefixSum[i] = prefixSum[i - 1] + arr[i]; 
}