pat1019数字黑洞，有一个测试点过不去，大神们帮我看下哪错了？谢谢

Question

 #include <stdio.h>
#include <stdlib.h>
int comp(const void *a,const void *b){
    return *(int*)a-*(int*)b;
}
int main(){
    int n,a[4]={0},i,x,y;
    scanf("%d",&n);
    do{
        i=0;
        while(n!=0){
            a[i]=n%10;
            n/=10;
            i++;
        }
        qsort(a,4,sizeof(int),comp);
        x=1000*a[3]+100*a[2]+10*a[1]+a[0];
        y=1000*a[0]+100*a[1]+10*a[2]+a[3];
        n=x-y;
        printf("%04d - %04d = %04d\n",x,y,n);
    }while(n!=6174&&n!=0);
    return 0;
}

Answer 1

http://blog.csdn.net/xtzmm1215/article/details/38444887