HDU 4380 Farmer Greedy [简单几何]

　　统计三角形内点的个数。

　　如下图所示，P(ABC)=|P(AB)+P(BC)-P(AC)|。P(AB)代表线段AB上方的点的个数

#include <string.h>
#include <stdio.h>
#include <algorithm>
#define MAXN 105
typedef long long LL;
struct pnt{
    LL x, y;
    bool operator < (const pnt &p) const {return x < p.x;}
}pn[MAXN], pm[MAXN*10];
LL xmult(pnt p1, pnt p2, pnt p3) {
    return (p2.x-p1.x)*(p3.y-p1.y) - (p2.y-p1.y)*(p3.x-p1.x);
}

int d[MAXN][MAXN], ans, cas = 1, n, m;
int main(){
    while (scanf("%d%d", &n, &m) != EOF) {
        for (int i = 0; i < n; i++) scanf("%I64d%I64d", &pn[i].x, &pn[i].y);
        for (int i = 0; i < m; i++) scanf("%I64d%I64d", &pm[i].x, &pm[i].y);
        memset(d, 0, sizeof d); ans = 0;
        std::sort(pn, pn + n);
        for (int i = 0; i < n; i++) for (int j = i+1; j < n; j ++) for (int k = 0; k < m; k++)
            if (pn[i].x <= pm[k].x && pm[k].x < pn[j].x && xmult(pn[i], pn[j], pm[k]) < 0) d[i][j]++;
        for (int i = 0; i < n; i++) for (int j = i+1; j < n; j ++) for (int k = i+1; k < j; k++)
            if (std::abs(d[i][j]-d[i][k]-d[k][j])&1) ans++;
        printf("Case %d: %d\n", cas++, ans);
    }
    return 0;
}