백준 1157 C++

#include <iostream>
#include <algorithm>
using namespace std;

int main(){
    string s;    
    cin >> s;

    int a[26]={0,};
    int max = 0;
    int cnt;
    int index;

    transform(s.begin(),s.end(),s.begin(),::toupper);

    for(int i=0; i<s.length(); i++){
        a[s[i] - 'A']++;
    }

    for(int i=0; i<26; i++){
        if(max < a[i]){
            max = a[i];
            cnt = 0;
            index = i;
        }
        if(max == a[i]){
            cnt++;
        }
    }

    if(cnt > 1) cout << "?";
    else cout << (char)(index + 'A');
    
    return 0;
}