QWERTY12345 YU
QWER TY4564 F
QWERGH145
QWER T777
QWE RBT

public static string CommonString(string first, string second)
{
    return new string((first.Intersect(second)).ToArray());
}

string common(const vector<string>& sts)
{
    for(int i = 0;;++i)
    {
        bool ok = true;
        for(int j = 1; j < sts.size(); ++j)
        {
            if (sts[j-1][i] != sts[j][i])
            {
                ok = false;
                break;
            }
        }
        if (ok == false) return sts[0].substr(0,i);
    }
}