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.
Credits:Special thanks to @mithmatt for adding this problem and creating all test cases.
利用堆排序的特性，依次找出最大值。
见程序注释