凯撒密码加密解密算法 C++

#include<stdio.h>
void main()
{
   char M[100];
   char C[100];
   int p;
   printf("请选择0:加密,1:解密\n");
   scanf("%d",&p);
   printf("请输入明文M（注意不要输入空白串）\n");
   scanf("%s",M);
   int i,K;
   printf("请输入密钥K（注意不要输入空白串）\n");
  scanf("%d",&K);
  if(p==0)
  {
 for(i=0;M[i]!='\0';i++)
 if(M[i]>='a'&&M[i]<='z')
  C[i]=(M[i]-'a'+K)%26+'a'; 
 else if(M[i]>='A'&&M[i]<='z')
  C[i]=(M[i]-'A'+K)%26+'A'; 
     C[i]='\0';
  }
 else if(p==1)
  {
    for(i=0;M[i]!='\0';i++)
 if(M[i]>='a'&&M[i]<='z')
  C[i]=(M[i]-'a'-K)%26+'a'; 
 else if(M[i]>='A'&&M[i]<='z')
  C[i]=(M[i]-'A'-K)%26+'A'; 
     C[i]='\0';