C_PRG_LANG_EX_4.13

#include <stdio.h>
#include <stdlib.h>

void reverse(char *s);
void reverse_rec(char *s, int left, int right);

void reverse(char *s)
{
    int end;
    for (end = 0; *(s + end);)
        end++;
    end--;

    reverse_rec(s, 0, end);
}

void reverse_rec(char *s, int left, int right)
{
    if (left >= right)
        return;

    char c = *(s + left);
    *(s + left) = *(s + right);
    *(s + right) = c;

    reverse_rec(s, left + 1, right - 1);
}

int main(void)
{
    char str[256] = "qweasd";
    reverse(str);
    printf("%s", str);
}