Efficiently remove duplicate white space from a string

//输入一个字符串，把连续的空格合并成一个。
void remove_duplicate_space(char *p)
{
    char *r=p, *w=p; //r: read, w: write
    while (*r != '\0') {
      if (*r!=' ' || *(w-1) != ' ')
      { *w = *r; w++; r++;}
      else
        r++;
    }
    if (r!=w) *w = '\0';
}