poj3348 Cows

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凸包板子

写凸包的要点:

极角排序就直接叉积 逆正顺负

算面积的时候三角形要/2.0

还有这题用整数就行了qwq

Code:

#include<cstdio>
#include<algorithm>
#include<cmath>
using namespace std;
typedef double D;
#define read(x) scanf("%lf",&x)
#define inf 2147483647
#define eps 1e-8
const int N = 100005;
#define rep(i,a,n) for(int i = a;i <= n;++i)
struct point {
    D x,y;
    point(){}
    point(int X,int Y):x(X),y(Y){}
    D len() {return sqrt(x*x+y*y);}
    point operator - (point o) {
        return (point){x-o.x,y-o.y};
    }
    D operator * (point o) {
        return x * o.y - y * o.x;
    }
}p[N],stk[N];
int top,n;

inline bool cmp(point a,point b) {
    D Cmp = (a - p[1]) * (b - p[1]);
    if(Cmp != 0.0) return Cmp > 0.0;
    return (a - p[1]).len() < (b - p[1]).len();
}

void Graham() {
    int id = 1;
    rep(i,2,n) if(p[i].x < p[id].x || (p[i].x == p[id].x && p[i].y < p[id].y)) id = i;
    swap(p[1],p[id]),sort(p+2,p+n+1,cmp);
    top = 0;
    stk[++top] = p[1];
    rep(i,2,n) {
        while(top >= 2 && (p[i] - stk[top-1]) * (stk[top] - stk[top-1]) >= 0.0) top--;
        stk[++top] = p[i];
    }
}

D S() {
    D res = 0.0;
    stk[top+1] = stk[1];
    rep(i,1,top) res += stk[i] * stk[i+1];
    return res / 2.0;
}

int main() {
    scanf("%d",&n);
    rep(i,1,n) scanf("%lf%lf",&p[i].x,&p[i].y);
    Graham();
    int ans = S() / 50;
    printf("%d
",ans);
    return 0;
}