题目见zoj 1006 或poj 1317
简单的解密算法，直接套用题目中公式即可。
/* zoj 1006 Do the Untwist */
#include <stdio.h>
#include <string.h>#define MAXLEN 80
#define MAGICNUM 28char num2Char(int n);
int char2Num(char c);
int main(void)
{int key;char ciphertext[MAXLEN],plaintext[MAXLEN];int ciphercode[MAXLEN],plaincode[MAXLEN];int slen,i;while(scanf("%d", &key) == 1 && key != 0){scanf("%s", ciphertext);slen = strlen(ciphertext);for(i = 0; i < slen; i++){ciphercode[i] = char2Num(ciphertext[i]);plaincode[(key * i) % slen] = (i+ciphercode[i])%MAGICNUM;}for(i = 0; i < slen; i++)plaintext[i] = num2Char(plaincode[i]);plaintext[slen] = '\0';printf("%s\n",plaintext);}return 0;
}
int char2Num(char c)
{if( c == '_')return 0;else if( c == '.')return 27;elsereturn c - 'a' + 1;
}
char num2Char(int n)
{if(n == 0)return '_';else if(n == 27)return '.';elsereturn 'a' + n - 1;
}
)
)
)
