#include <stdio.h>
#include <math.h>
void main()
{
    char c1,a[100],b[100],j;
    int i;
    gets(a);
    for(i=0;i<100&&(c1=a[i])!='\0';i++)
    {
        b[i]=a[i];
        if(c1>='A'&&c1<='Z')
        {
            j=c1-65+1;
            c1=c1+(26-j+1)-j;
        }
        else if(c1>='a'&&c1<='z')
        {
            j=c1-97+1;
            c1=c1+(26-j+1)-j;
        }
        printf("%c",c1);
    }
        printf("\n");
    for(i=0;i<100&&b[i]!='\0';i++)
        printf("%c",b[i]);
    printf("\n");
}

老大  你在b[i]中没有进行\0结尾  #include <stdio.h>
#include <math.h>
void main()
{
    char c1,a[100],b[100],j;
    int i;
    gets(a);
    for(i=0;i<100&&(c1=a[i])!='\0';i++)
    {
        b[i]=a[i];
        if(c1>='A'&&c1<='Z')
        {
            j=c1-65+1;
            c1=c1+(26-j+1)-j;
        }
        else if(c1>='a'&&c1<='z')
        {
            j=c1-97+1;
            c1=c1+(26-j+1)-j;
        }
        printf("%c",c1);
        b[i+1]='\0';
    }
        printf("\n");
    for(i=0;i<100&&b[i]!='\0';i++)
        printf("%c",b[i]);
    printf("\n");
}

#include <math.h>
void main()
{
    char c1,a[100],b[100],j;
    int i;
    gets(a);
    for(i=0;i<100&&(c1=a[i])!='\0';i++)
    {
        b[i]=a[i];
        if(c1>='A'&&c1<='Z')
        {
            j=c1-65+1;
            c1=c1+(26-j+1)-j;
        }
        else if(c1>='a'&&c1<='z')
        {
            j=c1-97+1;
            c1=c1+(26-j+1)-j;
        }
        printf("%c",c1);
      //  b[i+1]='\0';
    }
        printf("\n");
    for(i=0;i<100;i++)//&&b[i]!='\0'去掉，为什么还有乱码
        printf("%c",b[i]);
    printf("\n");
}

3Q  初学忽略了