Suppose LeetCode will start its IPO soon. In order to sell a good price of its shares to Venture Capital, LeetCode would like to work on some projects to increase its capital before the IPO. Since it has limited resources, it can only finish at most k distinct projects before the IPO. Help LeetCo...

Median is the middle value in an ordered integer list. If the size of the list is even, there is no middle value. So the median is the mean of the two middle value.

这道题目本质上是考察是否能将数据结构的知识灵活的运用于现实生活中。从最直观的想法来看，我们会有一个用户实体，每个用户会记录自己关注的用户的id，以及记录自己发表的所有tweet。这里唯一的难点在于我们如何按照时间顺序获取tweet流。

这里采用了两个优先队列来实现。一个优先队列用来存储字符流中较小的一半，另一个用来存储字符流中数值较大的一半。这样当需要获取当前中位数时，就可以根据当前的数值个数选择一个或是两个数的平均值。