func_code_string: #include <bits/stdc++.h>   using namespace std;   int main() {     int n;     string s;     cin >> n >> s;     vector<int> spareIndices;     vector<bool> LetterPresent(n);     for (int i = 0, j = s.length() - 1; i < (s.length() + 1) / 2; ++i, --j) {       if (s[i] != s[j]) {         if (s[i] ==  ? ) {           s[i] = s[j];           LetterPresent[s[i] -  a ] = true;           continue;         }         if (s[j] ==  ? ) {           s[j] = s[i];           LetterPresent[s[i] -  a ] = true;           continue;         }         cout <<  IMPOSSIBLE ;         return 0;       }       if (s[i] ==  ? ) {         spareIndices.push_back(i);         continue;       }       LetterPresent[s[i] -  a ] = true;     }     vector<char> unused_letters;     for (int i = 0; i < LetterPresent.size(); ++i) {       if (!LetterPresent[i]) unused_letters.push_back( a  + i);     }     if (unused_letters.size() > spareIndices.size()) {       cout <<  IMPOSSIBLE ;       return 0;     }     int a = spareIndices.size() - unused_letters.size();     int i = 0;     for (; i < a; ++i) {       s[spareIndices[i]] = s[s.size() - 1 - spareIndices[i]] =  a ;     }     for (; i < spareIndices.size(); ++i) {       s[spareIndices[i]] = s[s.size() - 1 - spareIndices[i]] =           unused_letters[i - a];     }     cout << s;   }   