func_code_string: #include <bits/stdc++.h>   using namespace std;   int n;   string s;   map<char, bool> m;   int main() {     cin >> n >> s;     char pl =  a ;     int sum = 0, sum2 = 0;     int len = s.size();     int book = 0;     for (int i = 0; i < len / 2; ++i) {       if (isalpha(s[i])) {         if (!m[s[i]]) {           m[s[i]] = 1;           sum2++;         }         if (s[len - i - 1] ==  ? ) {           s[len - i - 1] = s[i];         } else if (s[i] != s[len - i - 1]) {           puts( IMPOSSIBLE );           return 0;         }       }     }     for (int i = len / 2; i < len; ++i) {       if (isalpha(s[i])) {         if (!m[s[i]]) {           m[s[i]] = 1;           sum2++;         }         if (s[len - i - 1] ==  ? ) {           s[len - i - 1] = s[i];         } else if (s[i] != s[len - i - 1]) {           puts( IMPOSSIBLE );           return 0;         }       } else         sum++;     }     sum2 = n - sum2;     if (sum2 > sum) {       puts( IMPOSSIBLE );       return 0;     }     int tot = sum - sum2;     for (int i = 0; i <= len / 2; ++i) {       if (s[i] ==  ? ) {         if (tot) {           s[i] =  a ;           s[len - i - 1] =  a ;           tot--;         } else {           while (m[pl]) pl++;           s[i] = pl;           s[len - i - 1] = pl;           m[pl] = 1;         }       }     }     cout << s << endl;     return 0;   }   