Find the kth largest element in an unsorted array.
Note that it is the kth largest element in the sorted order,
not the kth distinct element.
For example,
Given [3,2,1,5,6,4] and k = 2, return 5.
Note:
You may assume k is always valid, 1 ≤ k ≤ array's length
https://www.cnblogs.com/grandyang/p/4539757.html