leetcode Top K Frequent Elements topK高频元素

Given a non-empty array of integers, 
return the k most frequent elements.
For example,
Given [1,1,1,2,2,3] and k = 2, return [1,2].
Note:
    You may assume k is always valid, 
    1 ≤ k ≤ number of unique elements.
    better than O(n log n), where n is the array's size.

http://www.cnblogs.com/grandyang/p/5454125.html