POJ1317 Do the Untwist

Do the Untwist

Time Limit: 1000MS		Memory Limit: 10000K
Total Submissions: 3424		Accepted: 2080

Description

Cryptography deals with methods of secret communication that transform a message (the plaintext) into a disguised form (the ciphertext) so that no one seeing the ciphertext will be able to figure out the plaintext except the intended recipient. Transforming
the plaintext to the ciphertext is encryption; transforming the ciphertext to the plaintext is decryption. Twisting is a simple encryption method that requires that the sender and recipient both agree on a secret key k, which is a positive integer. 

The twisting method uses four arrays: plaintext and ciphertext are arrays of characters, and plaincode and ciphercode are arrays of integers. All arrays are of length n, where n is the length of the message to be encrypted. Arrays are origin zero, so the elements
are numbered from 0 to n - 1. For this problem all messages will contain only lowercase letters, the period, and the underscore (representing a space). 

The message to be encrypted is stored in plaintext. Given a key k, the encryption method works as follows. First convert the letters in plaintext to integer codes in plaincode according to the following rule: '_' = 0, 'a' = 1, 'b' = 2, ..., 'z' = 26, and '.'
= 27. Next, convert each code in plaincode to an encrypted code in ciphercode according to the following formula: for all i from 0 to n - 1, 

ciphercode[i] = (plaincode[ki mod n] - i) mod 28. 

(Here x mod y is the positive remainder when x is divided by y. For example, 3 mod 7 = 3, 22 mod 8 = 6, and -1 mod 28 = 27. You can use the C '%' operator or Pascal 'mod' operator to compute this as long as you add y if the result is negative.) Finally, convert
the codes in ciphercode back to letters in ciphertext according to the rule listed above. The final twisted message is in ciphertext. Twisting the message cat using the key 5 yields the following: 

Array      0   1   2

plaintext 'c' 'a' 't'

plaincode  3   1   20

ciphercode 3   19  27

ciphertext 'c' 's' '.'

Your task is to write a program that can untwist messages, i.e., convert the ciphertext back to the original plaintext given the key k. For example, given the key 5 and ciphertext 'cs.', your program must output the plaintext 'cat'. 

Input

The input contains one or more test cases, followed by a line containing only the number 0 that signals the end of the file. Each test case is on a line by itself and consists of the key k, a space, and then a twisted message containing at least one and at
most 70 characters. The key k will be a positive integer not greater than 300.
Output

For each test case, output the untwisted message on a line by itself. 

Note: you can assume that untwisting a message always yields a unique result. (For those of you with some knowledge of basic number theory or abstract algebra, this will be the case provided that the greatest common divisor of the key k and length n is 1, which
it will be for all test cases.)
Sample Input

5 cs.
101 thqqxw.lui.qswer
3 b_ylxmhzjsys.virpbkr
0

Sample Output

cat
this_is_a_secret

beware._dogs_barking
题解：以为将一个字符串解码，题目中说明的是加密的规则，解码要反过来。
ciphercode[i] = (plaincode[ki mod n] - i) mod 28.注意ki为k*i
 plaincode[ki mod n] = (ciphercode[i]+i)mod 28.最后转换成对应的字母。
AC code:
#include <iostream>
#include <cstring>
#include <cstdio>

using namespace std;

int k;
int un[100];
char s[100], result[100];

int main()
{
while(scanf("%d", &k) && k)
{
scanf("%s", s);
int len = strlen(s);
for(int i = 0; s[i]!='\0'; i++)
{
int temp;
switch(s[i])
{
case '_':
temp = 0;
break;
case '.':
temp = 27;
break;
default:
temp = s[i] - 'a' + 1;
}
int j = (k * i) % len;
un[j] = (temp + i)%28;

if(un[j] == 0)
result[j] = '_';
else if(un[j] == 27)
result[j] = '.';
else
result[j] = (char)(un[j] + 'a' - 1);

}
result[len] = '\0';
printf("%s\n", result);
}
return 0;
}