很牛B，效率很高。

/*
Check if there are same charactors in
the string.And it may be the most
effective way.
*/

#include <stdio.h>

main(int argc,char *argv[]){
   puts(argv[1]);
   char num[256]=;   /*good way to init a char array*/
   unsigned char *pos = (unsigned char *)argv[1];
   while(*pos!=0&&num[*pos]==0){
      /*
        "*pos" used for checking if the string goes end.
        "num[*pos]==0" used for checking if the char appeared before.
      */
      num[*pos++]=1;
   }
   printf("%i\n",(*pos==0?0:1));
}

心得：

*用初始化数组。