
Given an integer array nums and an integer k , return the k most frequent elements . You may return the answer in any order .
1 <= nums.length <= 10 ^5-10 ^4 <= nums[i] <= 10 ^4k is in the range [1, the number of unique elements in the array] .It is guaranteed that the answer is unique .Follow up: Your algorithm's time complexity must be better than O(n log n) , where n is the array's size.