#include <stdio.h>
 
char * find_end_of_string(char * str)
{
    return *str ? find_end_of_string(str + 1) : str;
}
 
void do_reverse_string(char * a, char * b)
{
    char tmp;
 
    if (a < b) {
        tmp = *a;
        *a = *b;
        *b = tmp;
 
        do_reverse_string(a + 1, b - 1);
    }
}
 
void reverse_string(char * str)
{
    do_reverse_string(str, find_end_of_string(str) - 1);
}
 
int main() {
    char odd[] = "abcde";
    char even[] = "abcdef";
 
    reverse_string(odd);
    reverse_string(even);
 
    printf("%s\n%s\n", odd, even);
 
    return 0;
}

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