模板，凸包

int TuBao(P p[],int n,P q[]){
    int m = 0;
    sort(p,p+n);
    for(int i=0;i<n;++i) {
        while (m>1 && dcmp(cross(q[m-2],q[m-1],p[i]))<=0) m--;
        q[m++] = p[i];
    }
    int k = m;
    for(int i=n-2;i>=0;--i){
        while(m>k && dcmp(cross(q[m-2],q[m-1],p[i]))<=0) m--;
        q[m++] = p[i];
    }
    for(int i=0;i<m;++i) p[i] = q[i];
    if(n>1)m--;
    return m;
}