POJ 3348 Cows

POJ_3348

    先将凸包找到，然后再用叉积算出多边形的面积之后除以50就是结果。

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#define MAXD 10010
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;
    for(i = 0; i < N; i ++)
        scanf("%d%d", &p[i].x, &p[i].y);
    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;
    long long int ans = 0;
    if(N <= 2)
        printf("0\n");
    else
    {
        P = graham();
        for(i = 0; i < P; i ++)
            ans += det(res[i].x, res[i].y, res[i + 1].x, res[i + 1].y);
        ans = ans < 0 ? -ans : ans;
        printf("%lld\n", ans / 100);
    }
}
int main()
{
    while(scanf("%d", &N) == 1)
    {
        init();
        solve();
    }
    return 0;
}