Convex Hull Code

struct P{
    int x, y;
    bool operator<(const P &o) const{
        if (x != o.x){
            return x < o.x;
        }
        return y < o.y;
    }
};
int cross(const P &a, const P &b){
    return a.x * b.y - b.x * a.y;
}
int cross(const P &o, const P &a, const P &b){
    return cross({a.x - o.x, a.y - o.y}, {b.x - o.x, b.y - o.y});
}
int main(){
    vector<P> h;
    for (int i = 0; i < n; i++){
        while (h.size() > 1 && cross(h[h.size()-2],h.back(),a[i])<=0){
            h.pop_back();
        }
        h.push_back(a[i]);
    }
}