POJ 2007 Scrambled Polygon

POJ_2007

    这个题目本质上还是求凸包，然后按题目指定顺序输出各个点即可。

#include<stdio.h>
#include<string.h>
#include<math.h>
#include<stdlib.h>
#define MAXD 60
#define zero 1e-8
struct point
{
    int x, y;
}p[MAXD], res[MAXD];
int N, P;
int cmp(const void *_p, const void *_q)
{
    point *p = (point *)_p, *q = (point *)_q;
    if(p->y == q->y)
        return p->x - q->x;
    return p->y - q->y;
}
int det(int x1, int y1, int x2, int y2)
{
    return x1 * y2 - x2 * y1;
}
void init()
{
    int i, j, k;
    N = 0;
    while(scanf("%d%d", &i, &j) == 2)
        p[N].x = i, p[N].y = j, ++ N;
    qsort(p, N, sizeof(p[0]), cmp);
}
int del(int top, int i)
{
    if(det(res[top].x - res[top - 1].x, res[top].y - res[top - 1].y, p[i].x - res[top].x, p[i].y - res[top].y) < 0)
        return 1;
    return 0;
}
int graham()
{
    int i, j, k, top = 1, mint;
    res[0] = p[0], res[1] = p[1];
    for(i = 2; i < N; i ++)
    {
        while(top && del(top, i))
            -- top;
        res[++ top] = p[i];
    }
    mint = top;
    res[++ top] = p[N - 2];
    for(i = N - 3; i >= 0; i --)
    {
        while(top != mint && del(top, i))
            -- top;
        res[++ top] = p[i];
    }
    return top;
}
void solve()
{
    int i, j, k;
    P = graham();
    for(i = 0; ; i ++)
        if(res[i].x == 0 && res[i].y == 0)
            break;
    printf("(0,0)\n");
    for(j = (i + 1) % P; j != i; j = (j + 1) % P)
        printf("(%d,%d)\n", res[j].x, res[j].y);
}
int main()
{
    init();
    solve();
    return 0;
}