1456. Maximum Number of Vowels in a Substring of Given Length

class Solution {
public:
bool isvowel(char ch){
    return (ch=='a' || ch=='e' || ch=='i' || ch=='o' || ch=='u');
}
    int maxVowels(string s, int k) {
        int cnt=0;
        for(int i=0;i<k;i++){
            if(isvowel(s[i])) cnt++;
        }
        int idx=0,ans=cnt;
        for(int i=k;i<s.size();i++){
            if(isvowel(s[idx])){
                cnt--;
            }
            if(isvowel(s[i])) cnt++;
            ans=max(ans,cnt);
            idx++;
        }
        return ans;
    }
};