void simulate_anneal(){
    pair<double,double> cur={ansx,ansy};
    for(double t=3e4;t>1e-4;t*=0.996){
        pair<double,double> np(rand(cur.first-t,cur.first+t),rand(cur.second-t,cur.second+t));
        double dt=calc(np)-calc(cur);
        if(exp(dt/t)*RAND_MAX>(double)rand()){//这个判断方法为什么不对？
            cur=np;
        }
    }
}