#include <iostream.h>
#include <string.h>

void psw(char *strI,int t=0);  //加密公式函數,t=0加密 t=1解密
char getW(int i);  //字母表

void main()
{
 char str[80];
 int i;

 cout<<"1.加密\n";
 cout<<"2.解密\n";
 cout<<"3.退出\n";
 cout<<"選擇:";
 cin>>i;
 cout<<endl;
 
 switch(i)
 {
 case 1:
  cout<<"請輸入要加密的字符串:";
  cin>>str;
  psw(str);
  cout<<endl;
  break;
 case 2:
  cout<<"請輸入要解密的字符串:";
  cin>>str;
  psw(str,1);
  cout<<endl;
  break;
 default:
  break;
 }

}

void psw(char *strI,int t)
{
 char *p1;
 int i=0,k,s;
 s=t;
 p1=strI;
 while(strI[i]!='\0')
 {
  k=strI[i]-97;
  if(s)
  {
   cout<<getW((k+3)%26);
  }
  else
  {
   cout<<getW((k+23)%26);
  }
  i++;
 }
 cout<<endl;
}

char getW(int i)   
{
 char W[26];
 char h1;
 int j;
 j=i;
 h1=97;
 for(i=0;i<26;i++,h1++)
 {
  W[i]=h1;
 }
 return W[j];
}