POJ 3348 Cows 凸包 + 多边形面积公式

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一定要注意总的点数 n<= 2 时 graham不适用。

还要注意 graham找到凸包后注意 top <=2 时面积为0。

View Code

#include<stdio.h>#include<string.h>#include<math.h>#include<algorithm>using namespace std;struct point{    double x, y;}p[10003];double det(double x1, double y1, double x2, double y2){    return x1 * y2 - x2 * y1;}double cross(point o, point a, point b){     return det(a.x - o.x, a.y - o.y, b.x - o.x, b.y - o.y);}bool cmp(point a, point b){    if(a.y == b.y) return a.x < b.x;    return a.y < b.y;}int n;int top;point res[10003<<1];void graham(){    sort(p, p + n, cmp);    int i;    top = -1;    if(n <= 2 ) return;    res[++top] = p[0];    res[++top] = p[1];    for(i = 2; i < n; i++)    {        while(top >= 1 && cross(res[top-1], res[top], p[i]) <= 0 )            top--;        res[++top] = p[i];    }    res[++top] = p[n-2];    int t = top;    for(i = n - 3; i >= 0; i--)    {        while(top >= t && cross(res[top-1], res[top], p[i]) <= 0)            top--;        res[++top] = p[i];    }}double cal_area(int n, point* p){    double s = 0;    int i;    p[n] = p[0];    for (i = 0; i < n; i++)        s += p[i].x * p[i+1].y - p[i].y * p[i+1].x;    return fabs(s)/2;}int main(){    int i, j;    while( ~scanf("%d", &n) )    {        for(i = 0; i < n; i++)            scanf("%lf%lf", &p[i].x, &p[i].y);        graham();        if(top <= 2) { printf("0\n"); continue; }        double ans = cal_area(top, res);        printf("%d\n", (int) (ans/50));    }    return 0;}