大数（string 之间的快速幂）

 1 //字符串的乘法
 2 string multi(string a, string b){
 3     int i, j, arr[200], len = a.length() + b.length();
 4     memset(arr, 0, sizeof arr);
 5     reverse(a.begin(), a.end());
 6     reverse(b.begin(), b.end());
 7     for (int i = 0; i<a.length(); i++)
 8     for (int j = 0; j<b.length(); j++)
 9         arr[i + j] += (a[i] - ‘0‘)*(b[j] - ‘0‘);
10     for (int i = 0; i<len; i++){
11         arr[i + 1] += arr[i] / 10;
12         arr[i] %= 10;
13     }
14     string ret = string(len, ‘0‘);
15     for (int i = 0; i<len; i++)
16         ret[i] += arr[i];
17     reverse(ret.begin(), ret.end());//翻转
18     return ret;
19 }
20 
21 //x ^ n
22 string Str_Pow(string x, int p){
23     string ret = "1";
24     while (p){
25         if (p & 1) ret = multi(ret, x);
26         x = multi(x, x);
27         p >>= 1;
28     }
29     return ret;
30 }