凸包模板

struct point
{
    double x,y,angel;
} p[N],stack[N];
int top,n;

double dis(point a,point b)//求距离
{
    return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}

bool mult(point p1,point p2,point p0)//叉乘
{
    return (p1.x-p0.x)*(p2.y-p0.y) >= (p2.x-p0.x)*(p1.y-p0.y);
}

bool cmp(point a,point b)
{
    if(a.angel == b.angel)
    {
        if (a.x == b.x)
            return a.y > b.y;
        return a.x > b.x;
    }
    return a.angel < b.angel;
}

void graham()
{
//p为点集，n为点的个数，stack为凸包点集，top为凸包个数
    int i,k=0;
    point temp;
    for(i=0; i<n; i++)
        if(p[i].y<p[k].y||((p[i].y==p[k].y)&&(p[i].x<p[k].x)))
            k=i;
    temp=p[0];
    p[0]=p[k];
    p[k]=temp;
    for(i=1; i<n; i++)
        p[i].angel=atan2(p[i].y-p[0].y,p[i].x-p[0].x);
    sort(p+1,p+n,cmp);
    stack[0]=p[0];
    stack[1]=p[1];
    stack[2]=p[2];
    top=3;
    for(i=3; i<n; i++)
    {
        while(top > 2 && mult(stack[top-2],stack[top-1],p[i])<=0)
            top--;
        stack[top++]=p[i];
    }
}