最大/最小的k个数:
https://www.cnblogs.com/ceachy/archive/2013/03/20/Algorithm_Max_m_from_N.html
频率最多的k个数:
https://blog.youkuaiyun.com/weixin_38111819/article/details/79148354?spm=1001.2101.3001.6650.3&utm_medium=distribute.pc_relevant.none-task-blog-2%7Edefault%7ECTRLIST%7Edefault-3.opensearchhbase&depth_1-utm_source=distribute.pc_relevant.none-task-blog-2%7Edefault%7ECTRLIST%7Edefault-3.opensearchhbase