求循环小数

/* PRESET CODE BEGIN - NEVER TOUCH CODE BELOW */

#include <stdio.h>
#include <stdlib.h>

typedef struct node
{   int         data;
    struct node * next;
} NODE;

void output( NODE * );
void change( int, int, NODE * );

void output( NODE * head )
{   int k=0;

    printf("0.");
    while ( head->next != NULL && k<50 )
    {   printf("%d", head->next->data );
        head = head->next;
        k ++;
    }
    printf("\n");
}

void change(int n, int m, NODE * head)
{
    int a = n, b[50] = {0}, i=0, j=0;
    NODE * tmp = NULL;
    NODE * p = NULL, *q = head;
    do
    {
        b[i++] = a *= 10;
        for (p = head->next, j=0; p!=NULL && j<i-1; p=p->next)
        {
            if (b[j++] == a)
            {
                q->next = p;
                return;
            }
        }
        tmp = (NODE *)malloc(sizeof(NODE));
        tmp->data = a / m;
        a %= m;
        q->next = tmp;
        q = tmp;
        q->next = NULL;
    }
    while (a && i<50);
}

int main()
{   int n, m;
    NODE * head;

    scanf("%d%d", &n, &m);
    head = (NODE *)malloc( sizeof(NODE) );
    head->next = NULL;
    head->data = -1;
    change( n, m, head );
    output( head );
    return 0;
}