暴力循环计数即可

#include<bits/stdc++.h>
using namespace std;
int main(){
    string s;
    cin >> s;
    int maxx = -1;
    char ans;
    for(int i = 0;i < s.size();i++){
        int sum = 1;
        for(int j = 0;j < s.size();j++){
            if(s[i] == s[j]) sum++;
        }
        if(sum > maxx) {
            ans = s[i];
            maxx = sum;
        } else if(sum == maxx && s[i] < ans) ans = s[i];
    }
    cout << ans;
    return 0;
}