bool cmp(P p1,P p2){
    if(p1.x<p2.x) return 1;
    if(p1.x>p2.x) return 0;
    return p1.y<p2.y;
}
void andrew(){
    sort(s,s+n,cmp); 從0開始排序
    for(int i = 0;i < n;i ++){
        while(top>1&&cs(ch[top-2],ch[top-1],s[i])<=0) top --;
        ch[top++] = s[i];
    }
    int
 
 k = top;
    for(int i = n-2;i >= 0;i --){
        while(top>k&&cs(ch[top-2],ch[top-1],s[i])<=0) top --;
        ch[top++] = s[i];
    }
    if(n>1) top --;
}