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[], len = a.length() + b.length();
     memset(arr, , sizeof arr);
     reverse(a.begin(), a.end());
     reverse(b.begin(), b.end());
     for (int i = ; i<a.length(); i++)
     for (int j = ; j<b.length(); j++)
         arr[i + j] += (a[i] - '')*(b[j] - '');
     for (int i = ; i<len; i++){
         arr[i + ] += arr[i] / ;
         arr[i] %= ;
     }
     string ret = string(len, '');
     for (int i = ; 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 = "";
     while (p){
         if (p & ) ret = multi(ret, x);
         x = multi(x, x);
         p >>= ;
     }
     return ret;
 }