title: Find k missing elements in array of size n-k
body: <p>Given an array of <code>n</code> unique integers in [1, n], take random <code>k</code> elements away from the array, then shuffle it to be left with an array of size <code>n-k</code> of integers of size <code>n-k</code></p>
  <p>I want to find those <code>k</code> integers in the best complexity.</p>
  <p>If <code>k==1</code>, we can sum all the elements in the array, and the missing element would be the difference between <code>n(n+1)/2</code> (sum of all numbers from 1 to n) and the sum of the array elements.</p>
  <p>I would like to extend this algorithm to <code>k</code> missing elements by k equations, but don't know how to build the equations. Can it be done?</p>