1129 do the untwist.cpp

ACM 威士忌部分答案

#include <cstdio>
#include <string>
using namespace std;
char cipher[80];
char plain[80];

int main()
{
	int i,j,n,key,ccode,pcode,p;
	
	while( scanf("%d",&key)==1 && key )
	{
		scanf("%s",cipher);
		n=strlen(cipher);
		
		for(i=0;i<n;i++)
		{
			switch(cipher[i])
			{
			case '_':
				ccode=0;
				break;
			case '.':
				ccode=27;
				break;
			default :
				ccode=cipher[i]-'a'+1;
				break;
			}
			pcode=ccode+i;
			while(pcode>=28)//注意这儿 
				pcode-=28;
			p=key*i % n;
			switch(pcode)
			{
			case 0:
				plain[p]='_';
				break;
			case 27:
				plain[p]='.';
				break;
			default :
				plain[p]=pcode+'a'-1;
			}
		}
		plain[n]=0;
		printf("%s\n",plain);
	}
	return 0;
}