Mine

#include <stdio.h>

main()
{
        char str[30] = "this is a 30 character string\0";
        char *p;

        for (p = &str[29]; p != &str[0]; p--)
        {
                if (*p == ' ')
                {
                        *p = '\n';
                        break;
                }
        }

        puts(str);
        return 0;
}