This is my program.

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

typedef struct
{
    long int x, y;
    int u, id;
} snail;

snail s[1001];
int d[1002];
int n, k;

int cmp_snail(const void *a, const void *b)
{
    snail *c = (snail *)a;
    snail *d = (snail *)b;

    if(c -> x == d -> x) return c -> y - d -> y;
    return c -> x - d -> x;
}

int main(void)
{
    int i;

    memset(s, 0, sizeof(s));
    scanf("%ld%ld", &s[0] . x, &s[0] . y); s[0] . id = 0;
    scanf("%d", &n);

    for(i = 1; i <= n; i ++)
    scanf("%ld%ld%d", &s[i] . x, &s[i] . y, &s[i] . id);

    qsort(s, n + 1, sizeof(snail), cmp_snail);

    d[0] = 0; k = 1;
    s[0] . u = 1;

    for(i = 1; i <= n; i ++)
    {
    int find = 1;
    d[k ++] = i;
    while(find && k > 2)
    {
        long int x1 = s[d[k - 3]] . x; long int y1 = s[d[k - 3]] . y;
        long int x2 = s[d[k - 2]] . x; long int y2 = s[d[k - 2]] . y;
        long int x  = s[d[k - 1]] . x; long int y  = s[d[k - 1]] . y;
        long int a = y1 - y2;
        long int b = x2 - x1;
        long int c = y2 * x1 - y1 * x2;
        long int v = a * x + b * y + c;

        if(v > 0)
        {
        d[k - 2] = d[k - 1];
        k --;
        } else if(v == 0)
        {
        printf("-1\n");
        return 0;
        } else find = 0;
    }
    }
    printf("k = %d; %d\n", k, s[0] . id);

    for(i = 0; i < k; i ++)
    s[d[i]] . u = 1;

    for(i = n; i >= 0; i --)
    if(!s[i] . u) d[k ++] = i;

    k = -1;
    for(i = 0; i <= n && k == -1; i ++)
    if(s[d[i]] . id == 0) k = i;

    for(i = 0; i <= n; i ++)
    {
    printf("%d\n", s[d[k]] . id);
    k ++;
    k %= n + 1;
    }
    printf("0\n");
}

I tested it using many random tests. I tested it usign GRX from djgpp
and all seemed OK. Why do I get WA?

     for(i = 0; i <= n; i ++)
     {
     printf("%d\n", s[d[k]] . id);
/**     k ++;
     k %= n + 1;      **/
        k = (k+1)%(n+1);
     }
     printf("0\n");


I don't check very well your program, but at first glance it's look
ok, but a detail. I argumented your program, you'll see isn't the
same. Good Luck.

>      for(i = 0; i <= n; i ++)
>      {
>      printf("%d\n", s[d[k]] . id);
> /**     k ++;
>      k %= n + 1;      **/
>         k = (k+1)%(n+1);
>      }
>      printf("0\n");
>
>
> I don't check very well your program, but at first glance it's
look
> ok, but a detail. I argumented your program, you'll see isn't the
> same. Good Luck.

No, it didn't work. But thanks for thinking about.