Point stk[maxn];
int top;

bool cmpYX(const Point A,const Point B)//按Y-X排序
{
    if(A.y<B.y) return true;
    if(A.y==B.y){
        return A.x<=B.x;
    }
    return false;
}

bool cmp(const Point A,const Point B)//按极角排序
{
    return (A-p[0])*(B-p[0])>EPS;
}

void graham_scan()
{
    sort(p,p+N,cmpYX);
    sort(p+1,p+N,cmp);
    top=0;//用数组模拟栈
    stk[top++]=p[0];
    stk[top++]=p[1];
    for(int i=2;i<N;i++){
        if((stk[top-1]-stk[top-2])*(p[i]-stk[top-1])<-EPS){
            top--;
            stk[top++]=p[i];
        }
        else stk[top++]=p[i];
    }
}