B 90求 debug

#include <bits/stdc++.h>
using namespace std;
int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    string s;
    cin >> s;
    int n = s.size();
    s = " " + s;
    int cnt = 0;
    for (int i = 1; i <= n / 2; i++) {
        int j = n - i + 1;
        if (s[i] != s[j]) cnt++;
    }
    if (cnt == 0) {
        bool flag = true;
        for (int i = 1; i <= n / 2; i++) {
            if (s[i] != 'a') {
                s[i] = s[n - i + 1] = 'a';
                flag = false;
                break;
            }
        }
        if (flag && n % 2 == 1) s[n / 2 + 1] = 'a';
    } else if (cnt == 1) {
        for (int i = 1; i <= n / 2; i++)
            if (s[i] != s[n - i + 1])
                s[i] = s[n - i + 1] = 'a';
    } else {
        for (int i = 1; i <= n / 2; i++)
            if (s[i] != s[n - i + 1])
                s[i] = s[n - i + 1] = min(s[i], s[n - i + 1]);
    }
    s = s.substr(1);
    cout << s << endl;

    return 0;
}

自我感觉分类讨论全了，但是 WA on #9