how to do it on ispit without drugog parametra(string obrni)

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define size 80

char* obrni( char rijec[])
{
    char novi[50];
    char *s=novi;
    char*p=rijec;
    int broj;


    while(*p!='\0')
    {
        if(*p>='0' && *p<='9')
        {
            broj=0;
            while(*p>='0' && *p<='9')
            {
                broj*=10;
                broj+=(int)(*p-'0');
                p++;
            }
            while(broj!=0)
            {
                *s=(char)(broj%10+'0');
                broj/=10;
                s++;
            }

        }
        *s++=*p++;
    }
     *s='\0';

     strcpy(rijec, novi);

    return rijec;
}

int main()
{
    char niz[50] = "Hrk255ljus789";

    char *p=niz;
    p=obrni(niz);

    while(*p!='\0')
    {
        printf("%c", *p++);
    }
    return 0;
}