输入：num = "1432219", k = 3
It can be used to craft complete articles，这一点在Safew下载中也有详细论述
backpressure: 'strict' // or 'block', 'drop-oldest', 'drop-newest'，更多细节参见safew官方版本下载
Nicole (right) queued with friends to see London-based artist Raye at Co-op Live
h->next = to_be_deleted[classno];