凸包模板（安德鲁）

struct point
{
    double x,y;
}a[Max];
bool cmp(point a,point b)
{
    if(a.x!=b.x)
    return a.x<b.x;     //按照x从小到大排序
    return a.y<b.y;     //x相同按照y
}
int chaji(point a,point b,point c)
{
    a.x=c.x-a.x;
    a.y=c.y-a.y;
    b.x=c.x-b.x;
    b.y=c.y-b.y;
    if(a.x*b.y-b.x*a.y<1e-9) return 1;   //如果叉积小于0在顺时针方向
    return 0;
}
void solve()
{
    vector <point> up;  //构件凸包上部
    vector <point> down; //构造凸包下部
    up.push_back(a[0]);
    up.push_back(a[1]);
    down.push_back(a[n-1]);
    down.push_back(a[n-2]);
    for(int i=2;i<n;i++)
    {
        for(int j=up.size();j>=2&&chaji(up[j-2],up[j-1],a[i])!=1;j--)
            up.pop_back();  //将不能构成顺时针的去掉
        up.push_back(a[i]);  //重新加入一个点
    }
    for(int i=n-3;i>=0;i--)
    {
        for(int j=down.size();j>=2&&chaji(down[j-2],down[j-1],a[i])!=1;j--)
            down.pop_back();
        down.push_back(a[i]);
    }
    for(int i=down.size()-2;i>=1;i--) up.push_back(down[i]);
}
int m