title: Missing integer variation - O(n) solution needed
body: <p>The problem comes from Codility programming training and it sounds as follows:
  we have an array (A[]) with n (ranging from 1 to 100,000) elements and these are our parameters. The elements of the array  are integers from −2,147,483,648 to 2,147,483,647, and we need to find smallest positive integer that is NOT in the array. Of course this could be done easily in O(n*log n) by sorting them all and going through the sorted array, looking for the missing posiitve number (this last operation has O(n) worst time complexity in my solution). But according to Codility, this ENTIRE problem can be done in O(n), and I cannot see any way to do that. Could someone give some tips to let me get un-stuck?</p>
  
  <p>PS Here is a link to detailed description of the problem which I'm not allowed to copy - <a href="https://codility.com/c/intro/demo35UEXH-EAT" rel="noreferrer">https://codility.com/c/intro/demo35UEXH-EAT</a></p>