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>0)return 1;if(x<0)return 0;
    return Dis(O,A)<Dis(O,B);
}

int Graham(D*R,int n,D*A){
    for(int i=1;i<=n;i++)
        if(R[i].y<R[1].y||(R[i].y==R[1].y&&R[i].x<R[1].x))swap(R[i],R[1]);
    O=R[1],sort(R+2,R+n+1,acomp);
    int m=0;A[++m]=R[1],A[++m]=R[2];
    for(int i=3;i<=n;i++){
        while(m>1 && Cross(A[m-1]-R[i],A[m]-R[i])<=0)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