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[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;
}
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;
}
int main(){
    string a;
    int n;
    while (cin >> a >> n){
        int index = a.find('.');
        if (index == -1) index = 0;
        else {
            //substr() 是字符串截取函数
            a = a.substr(0, index) + a.substr(index + 1);
            index = (a.length() - index)*n;
        }
        a = Str_Pow(a, n);
        a = a.substr(0, a.length() - index) + "." + a.substr(a.length() - index);
        int i;
        for (i = 0; i<a.length(); i++)
            if (a[i] != '0')
                break;
        //substr(i) 从i到end
        a = a.substr(i);
        for (int i = a.length() - 1; i >= 0; i--){
            if (a[i] == '.'){
                a = a.substr(0, i); 
                break;
            }
            else if (a[i] != '0'){
                a = a.substr(0, i + 1);
                break;
            }
        }
        cout << a << endl;
    }
    return 0;
}