杭电OJ 1129（C++）

#include <iostream>
#include <cstring>
using namespace std;

const int MAXN = 100;

int character[28] = { '_','a','b','c','d','e','f','g','h','i','j','k','l','m',
                      'n','o','p','q','r','s','t','u','v','w','x','y','z','.' };
char plaincode[MAXN]; //原文
char ciphercode[MAXN]; //密文
int n, k;

int index(char ch) //查找字符ch在字符表中的位置
{
    for (int i = 0; i < 28; i++)
    {
        if (character[i] == ch)
            return i;
    }
}

int main()
{
    while (cin >> k)
    {
        if (k == 0)
            break;
        memset(plaincode, 0, sizeof(plaincode));
        memset(ciphercode, 0, sizeof(ciphercode));
        cin >> ciphercode;
        n = strlen(ciphercode); //密文长度
        
        for (int i = 0; i < n; i++) //解密密文
        {
            plaincode[(i * k) % n] = character[(index(ciphercode[i]) + i) % 28];
        }
        cout << plaincode << endl;
    }
    return 0;
}