求凸包面積
阿新 • • 發佈:2019-02-02
#include<iostream> #include<cstdio> #include<cstring> #include<cmath> #include<algorithm> using namespace std; struct Point{ double x,y; Point(double x=0,double y=0):x(x),y(y){} }; typedef Point Vector; bool cmp_x(const Point &p,const Point &q) { if(p.x!=q.x) return p.x<q.x; return p.y<q.y; } Vector operator -(Point A,Point B){return Vector(A.x-B.x,A.y-B.y);} Point p[50010],ch[50010]; int Cross(Vector A,Vector B){return A.x*B.y-A.y*B.x;} int ConvexHull(Point *p,int n,Point *ch) { sort(p,p+n,cmp_x); int m=0; for(int i=0;i<n;i++){ while(m>1&&Cross(ch[m-1]-ch[m-2],p[i]-ch[m-2])<=0)m--; ch[m++]=p[i];} int k=m; for(int i=n-2;i>=0;i--) { while(m>k&&Cross(ch[m-1]-ch[m-2],p[i]-ch[m-2])<=0)m--; ch[m++]=p[i]; } if(n>1) m--; return m; } double ConvexPolygonArea(Point *ch,int m) { double area=0; for(int i=1;i<m-1;i++) area+=Cross(ch[i]-ch[0],ch[i+1]-ch[0]); return area/2.0; } int main() { int n; while(scanf("%d",&n)!=EOF) { memset(p,0,sizeof(p)); memset(ch,0,sizeof(ch)); for(int i=0;i<n;i++) scanf("%lf%lf",&p[i].x,&p[i].y); int m=ConvexHull(p,n,ch); double area=ConvexPolygonArea(ch,m); printf("%.2lf\n",area); } return 0; }