215. Kth Largest Element in an Array

Find thekth 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.

Analysis:

Approach#1: maxheap, add all elements, poll k number of element out, return.

Approach#2: quick sort, keep pick random(usually pick end element) element as pivot to do partition. when partition position is not k. partition it again for just part of the array until position is k.