#include <stdio.h>

char* reverse_strcat(char *dest, const char *source);

int main (void)
{
  char d[20] = "ABC";
  char s[20] = "WXYZ";
  reverse_strcat(d, s);
  printf("%s\n", d);
  return 0;
}

char* reverse_strcat(char *dest, const char *source)
{
  const char* p1 = source;

  while (*dest != '\0') dest++;
  while (*p1 != '\0') p1++;
  while ((p1 - source) > 0) *(dest++) = *(--p1);
  *dest = '\0';

  return dest;
}