#include <cstdlib>
#include <ctime>
#include <string>

std::string randomstring(int len) {
    srand((long)time(0));
    std::string s;
    for(int i=0; i<len; ++i) {
        int rand = rand() % 3;
        s.append(
            rand == 0 ?
                s = rand()%(90-65) + 65 //A-Z
            : rand == 1 ?
                s = rand()%(57-48) + 48 //0-9
            : rand()%(122-97)+97 //a-z
        );
    }
    return s;
}