func_code_string: #include <bits/stdc++.h>   using namespace std;   int main() {     istream& in = cin;     int n;     in >> n;     vector<string> v;     for (int i = 0; i < n; i++) {       string str;       in >> str;       v.push_back(str);     }     set<char> use;     map<char, set<char> > use2;     for (int i = 0; i < n; i++) {       char p, c;       for (int j = 0; j < v[i].length(); j++) {         c = v[i][j];         use.insert(c);         if (j != 0) {           use2[p].insert(c);         }         p = c;       }     }     if (use.size() != 26) {       for (char ch =  a ; ch <=  z ; ch++) {         if (use.find(ch) == use.end()) {           cout << ch << endl;           break;         }       }     } else {       bool find = false;       for (char c1 =  a ; c1 <=  z  && !find; c1++) {         set<char> s = use2[c1];         for (char c2 =  a ; c2 <=  z  && !find; c2++) {           if (s.find(c2) == s.end()) {             cout << c1 << c2 << endl;             find = true;           }         }       }     }     return 0;   }   