计算几何-Graham法-凸包

This article is made by Jason-Cow.
Welcome to reprint.
But please post the article's address.

关键一句话

Cross(A[m-1]-R[i],A[m]-R[i])<=0)m--

附加，极角排序，课上讲~~

 D O;
 bl acomp(D A,D B){
     db x=Cross(A-O,B-O);
     if(x>)return ;if(x<)return ;
     return Dis(O,A)<Dis(O,B);
 }

 int Graham(D*R,int n,D*A){
     for(int i=;i<=n;i++)
         if(R[i].y<R[].y||(R[i].y==R[].y&&R[i].x<R[].x))swap(R[i],R[]);
     O=R[],sort(R+,R+n+,acomp);
     int m=;A[++m]=R[],A[++m]=R[];
     for(int i=;i<=n;i++){
         while(m> && Cross(A[m-]-R[i],A[m]-R[i])<=)m--;
         A[++m]=R[i];
     }
     return m;
 }

学了Andrew后，觉得Graham好丑，而且常数还大

唯一的好处是好写，其实差不多