Did you try using c++ std::sort(ar.begin(), ar.end()); ? I did and it worked flawlessly.

int birthdayCakeCandles(int n, vector<int> ar) {
// Complete this function
sort(ar.begin(), ar.end());
int max = ar[ar.size() -1];
int count = 0;
for(auto num : ar) {
    if(num == max) ++count;
}
return count;

}