zju1006.cpp

zju 1006 Do the Untwist

#include <iostream>
#include <string>
using namespace std;
int main()
{
	int k, l, i, plaincode[70], ciphercode[70];
	char plaintext[70], ciphertext[70];
	while( cin >> k, k)
	{
		cin >> ciphertext;
		l = strlen(ciphertext);
		for( i = 0; i < l; ++i)
		{
			ciphercode[i] = ciphertext[i]-'a'+1;
			if (ciphertext[i]=='.') ciphercode[i]=27;
			if (ciphertext[i]=='_') ciphercode[i]=0;
		}
		for( i = 0; i < l; ++i)
			plaincode[ k*i % l] = (ciphercode[i] + i) % 28;
		for( i = 0; i < l; ++i)
		{
			plaintext[i] = plaincode[i]+'a'-1;
			if (plaincode[i]==27) plaintext[i]='.';
			if (plaincode[i]==0) plaintext[i]='_';
		}
		for( i = 0; i < l; ++i)
			cout<<plaintext[i];
		cout<<endl;
	}
	return 0;
}