#include <stdio.h>
#include <string.h>

void invert (char s[]) {
	int anz = 'a' - 'A';
	int i;
	for (i = 0; i < strlen(s); i++) {
		int c = s[i];
		if (isupper(c)) {
			c = c + anz;
		} else if (islower(c)) {
			c = c - anz;
		}
		s[i] = c;
	}
}

int main() {
	char s[] = "Hallo Welt, ich bin's!";
	printf("%s\n", s);
	invert(s);
	printf("%s\n", s);
	invert(s);
	printf("%s\n", s);
}