http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=10018

toggling_cases.c

#include <stdio.h>
int check(char c)
{
    if ('A'<= c && c <='Z') return 1;
    else if ('a'<=c && c<='z') return 0;
    else return 2;
}
/** メイン関数 **/    
int main(void)
{
    char c;
    while (scanf("%c", &c), c != '\n')
        printf("%c", (check(c)==1) ? c+' ' : (check(c)?c:c-' '));
    putchar('\n');
    return 0;
}