51Nod 1873 初中的算术

大神的字符串快速幂

 #include <iostream>
 #include <string>
 #include <algorithm>
 #include <cstdio>
 #include <cstring>
 using namespace std;

 //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;
 }
 string Str_Pow(string x, int p){
     string ret = "";
     while (p){
         if (p & ) ret = multi(ret, x);
         x = multi(x, x);
         p >>= ;
     }
     return ret;
 }
 int main(){
     string a;
     int n;
     while (cin >> a >> n){
         int index = a.find('.');
         if (index == -) index = ;
         else {
             //substr() 是字符串截取函数
             a = a.substr(, index) + a.substr(index + );
             index = (a.length() - index)*n;
         }
         a = Str_Pow(a, n);
         a = a.substr(, a.length() - index) + "." + a.substr(a.length() - index);
         int i;
         for (i = ; i<a.length(); i++)
             if (a[i] != '')
                 break;
         //substr(i) 从i到end
         a = a.substr(i);
         for (int i = a.length() - ; i >= ; i--){
             if (a[i] == '.'){
                 a = a.substr(, i);
                 break;
             }
             else if (a[i] != ''){
                 a = a.substr(, i + );
                 break;
             }
         }
         cout << a << endl;
     }
     return ;
 }