func_code_string: #include <bits/stdc++.h>   using namespace std;   int main() {     std::ios::sync_with_stdio(false);     string s, res =   ;     cin >> s;     int len = s.length();     if (len <= 3) {       cout << s[0] << endl;       return 0;     }     int l = 0, r = len - 1;     while (l + 1 < r - 1) {       if (s[l] == s[r] || s[l] == s[r - 1])         res += s[l];       else if (s[l + 1] == s[r] || s[l + 1] == s[r - 1])         res += s[l + 1];       l += 2, r -= 2;     }     string ch =   ;     if (l <= r) ch += s[l];     string re_res = res;     reverse(re_res.begin(), re_res.end());     cout << res << ch << re_res << endl;     return 0;   }   