计算几何-Andrew法-凸包

This article is made by Jason-Cow.
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利用一下叉积和栈

 int Andrew(D*R,int n,D*A){
     int m=;
     sort(R+,R+n+);//for(int i=1;i<=n;i++)cout<<R[i].x<<" "<<R[i].y<<endl;
     for(int i=;i<=n;i++){
         while(m>= && Cross(A[m]-A[m-],R[i]-A[m-])<=)m--;
         A[++m]=R[i];
     }
     int k=m;
     for(int i=n-;i>=;i--){
         while(m>k  && Cross(A[m]-A[m-],R[i]-A[m-])<=)m--;
         A[++m]=R[i];
     }
     return n>?m-:m;
 }