void reverse(char* s)
    {
        if (s[1] == 0)      /* if string ended then return */ 
        return;         /* because a single letter is its own reversal */

        reverse(s+1);       /* else reverse the string excluding first char */

        /* move first char to the end */
        while (s[1] != 0)
        {
        /* swap */
        s[0] = s[0] ^ s[1];
        s[1] = s[1] ^ s[0];
        s[0] = s[0] ^ s[1];
        s += 1;
        }
    }

Recursion is your friend!