Longest sub array with sum equal to zero

Given an array of integers, find the longest continuous sub array which sum equals to zero

 

vector LongestSum0(const vector &A)
{
   vector output(2);
   unordered_map sumIndexMap;
   int n = A.size();
   long long sum = 0;
   int maxLen = 0;   

   if (n < 1) return output;
   for(int i = 0; i < n; i++) {
       sum += A[i];
       if (sum == 0) {
          maxLen = i + 1;
          output[0] = 0;
          output[1] = i;
       }
       else if (sumIndexMap.find(sum) != sumIndexMap.end()) { 
          if (i - sumIndexMap[sum] + 1 > maxLen) {
              maxLen = i - sumIndexMap[sum] + 1;
              output[0] = sumIndexMap[sum] ;
              output[1] = i;
          }
       }
       else  {
          sumIndexMap[sum] = i;
       }      
   }

   return output;
}