大数（string 之间的快速幂）

//字符串的乘法
string multi(string a, string b){
    int i, j, arr[200], len = a.length() + b.length();
    memset(arr, 0, sizeof arr);
    reverse(a.begin(), a.end());
    reverse(b.begin(), b.end());
    for (int i = 0; i<a.length(); i++)
    for (int j = 0; j<b.length(); j++)
        arr[i + j] += (a[i] - '0')*(b[j] - '0');
    for (int i = 0; i<len; i++){
        arr[i + 1] += arr[i] / 10;
        arr[i] %= 10;
    }
    string ret = string(len, '0');
    for (int i = 0; i<len; i++)
        ret[i] += arr[i];
    reverse(ret.begin(), ret.end());//翻转
    return ret;
}

//x ^ n
string Str_Pow(string x, int p){
    string ret = "1";
    while (p){
        if (p & 1) ret = multi(ret, x);
        x = multi(x, x);
        p >>= 1;
    }
    return ret;
}