func_code_string: #include <bits/stdc++.h>   #pragma comment(linker,  /STACK:16777216 )   using namespace std;   const double PI = 2.0 * acos(0.0);   const int MAXN = 33;   int n, m;   string s;   bool d1[26];   bool d2[700];   int encode(char x, char y) { return (x -  a ) * 26 + (y -  a ); }   int main() {     memset(d1, false, sizeof(d1));     memset(d2, false, sizeof(d2));     cin >> n;     for (int i = 0, _n = (n); i < _n; i++) {       cin >> s;       m = s.length();       for (int j = 0, _n = (m); j < _n; j++) {         d1[s[j] -  a ] = true;         if (j + 1 < m) d2[encode(s[j], s[j + 1])] = true;       }     }     char r1, r2;     int x;     for (int i = 0, _n = (26); i < _n; i++) {       if (!d1[i]) {         r1 =  a  + i;         cout << r1 << endl;         return 0;       }     }     for (int i = 0, _n = (700); i < _n; i++) {       if (!d2[i]) {         r1 =  a  + (i / 26);         r2 =  a  + (i % 26);         cout << r1 << r2 << endl;         return 0;       }     }   }   