C++ solution (to pass all test cases)
int numberOfAlerts(int precedingMinutes, int alertThreshold, vector numCalls) {
int res = 0;
double total = 0.0;
double precedingMinutes_ = (double) precedingMinutes;
for(int i =0;i<precedingMinutes;i++){
total = total + (double)numCalls[i];
}
if(total/precedingMinutes_ > alertThreshold){
res++;
}
for(int i =precedingMinutes;i<numCalls.size();i++){
total = total + (double)numCalls[i];
total = total - (double)numCalls[i - precedingMinutes];
if(total/precedingMinutes_ > alertThreshold){
res++;
}
}
return res;
}