To reverse the order of two words ..... if I enter "hello world" , I should get "world hello "

#include<stdio.h>
void reverse(char *begin, char *end);

void reverseWords(char *s)
{
  char *word_begin = s;
  char *temp = s;

  while( *temp )
  {
    temp++;
    if (*temp == '\0')
    {
      reverse(word_begin, temp-1);
    }
    else if(*temp == ' ')
    {
      reverse(word_begin, temp-1);
      word_begin = temp+1;
    }
  }
  reverse(s, temp-1);
}

void reverse(char *begin, char *end)
{
  char temp;
  while (begin < end)
  {
    temp = *begin;
    *begin++ = *end;
    *end-- = temp;
  }
}

int main()
{
  char *s;
  gets(s);
  char *temp = s;
  reverseWords(s);
  printf("%s", s);
  getchar();
  return 0;
}