通過鍵盤輸入一串小寫字母(a~z)組成的字符串。請編寫一個字符串過濾程序，若字符串中出現多個相同的字符，將非首次出現的字符過濾掉

#include <stdio.h>

void char_filter(char *elem);

int main( )
{
    char elem[255];
    char *data;

    data =elem;

    printf("請輸入字符串：\n");

    gets(elem);

    char_filter(elem);

    printf("處理後的字符串爲：\n");

    puts(elem);

    return 0;
}

void char_filter(char *elem)
{
    char data;
    int i, j, k;
    for(i=0; elem[i]!='\0';i++)
    {
        data = elem[i];
        k = i+1;
        for(j=i+1; elem[j]!='\0'; j++)
        {
            if( data != elem[j] )
            {
                elem[k] = elem[j];
                k++;
            }
        }
        elem[k] = '\0';
    }
}