ข้อ 2 นะ

#include<stdio.h>

char char_twist(char c);
char char_next(char c);
int main()
{
    char c;
    do
    {
        c = getchar();
        putchar(char_twist(char_next(c)));
    }while(c != EOF);

    return 0;
}

char char_twist(char c)
{
    if ((c >= 'A' && c <= 'Z')||(c >= 'a' && c <= 'z'))
    {
        if(islower(c))
            return toupper(c);
        else
            return tolower(c);
    }
    return c;
}

char char_next(char c)
{
    if ((c >= 'A' && c < 'Z')||(c >= 'a' && c < 'z'))
        return ++c;
    else if (c == 'Z' || c == 'z')
        return c - 25;
    return c;
}