1704 Determine if string halves are alike

class Solution {
public:
    bool halvesAreAlike(string s) {
       unordered_set<char> vowels{'a','e','i','o','u','A','E','I','O','U'};
      
        int mid=s.size()/2;
        int count_a=0,count_b=0;
        for (int i=0; i<mid; ++i) {
            if (vowels.count(s[i])) {
                ++count_a;
          }
            if (vowels.count(s[mid+i])) {
                ++count_b;
          }
        }
      return count_a == count_b; 
    }
};