POJ ( acm.pku.edu.cn ) 1026

#include <iostream>
#include <string.h>
#include <algorithm>
using namespace std;
//void encode ( char* in, char* out, int* key, int len, int k) {
//    for (;k;--k){
//        for (int i=0;i<len;++i) {
//            out[key[i]]=in[i];
//        }
//        swap(in,out);
//    }
//    swap(in,out);
//}

char* encode(char* in, char* out, int* key, int len, int k) {
for (int i=0;i<len;++i){
   int n,c,a;
   for(n=1,c=key[i];c!=i;c=key[c])++n;
   for(c=i,a=k%n;a;--a)c=key[c];
   out[c]=in[i];
}
return out;
}

int main() {
for (int N;cin>>N&&N;){
   int key[256];
   for(int i=0;i<N;++i) {
       cin>>key[i];
       --key[i];
   }
   for(int k;cin>>k&&k>0;){
       cin.get();//space
       char msg[256],enmsg[256]={0}; cin.getline(msg,256);
       int l=strlen(msg);
       for(;l<N;++l)msg[l]=' ';
       msg[l]=0;
       encode(msg,enmsg,key,N,k);
       cout<<enmsg<<endl;
   }
   cout<<endl;
}
return 0;
}

之后，为了避免重复计算，我这么改进了：

#include <iostream>
#include <string.h>
#include <algorithm>
using namespace std;

class encoder {
 int* key;
 int len;
 int circle_len[256];
 encoder();
 encoder(encoder const&);
public:
 encoder(int *key, int len):key(key),len(len){calc_circle();}
 ~encoder(){delete[] circle_len;}
 void calc_circle() {
     for (int i=0;i<len;++i){
         int n,c;
         for(n=1,c=key[i];c!=i;c=key[c])++n;
         circle_len[i]=n;
     }
 }
 char* operator()(char* in, char* out, int k){
     for (int i=0;i<len;++i){
         int c,a;
         for(c=i,a=k%circle_len[i];a;--a)c=key[c];
         out[c]=in[i];
     }
     return out;
 }
};


int main() {
 for (int N;cin>>N&&N;){
     int key[256];
     for(int i=0;i<N;++i) {
         cin>>key[i];
         --key[i];
     }
     encoder encode(key, N);
     for(int k;cin>>k&&k>0;){
         cin.get();//space
         char msg[256],enmsg[256]; cin.getline(msg,256);
         int l=strlen(msg);
         for(;l<N;++l)msg[l]=' ';
         msg[l]=enmsg[l]=0;
         encode(msg,enmsg,k);
         cout<<enmsg<<endl;
     }
     cout<<endl;
 }
 return 0;
}

——结果是戏剧性的 Runtime Error