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All leaked interview problems are collected from Internet.

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. - Your algorithm's time complexity
**must be**better than O(*n*log*n*), where*n*is the array's size.

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