//关键代码
    //维护单调递减队列 
        if (q1[head1] + k <= i)
            head1++;
        while (head1 <= tail1 && a[q1[tail1]] <= a[i])
            tail1--;
        q1[++tail1] = i;
        //维护单调递增队列 
        if (q2[head2] + k <= i)
            head2++;
        while (head2 <= tail2 && a[q2[tail2]] >= a[i])
            tail2--;
        q2[++tail2] = i;