title: find pair of numbers in array that add to given sum
body: <p>Question: Given an unsorted array of positive integers, is it possible to find a pair of integers from that array that sum up to a given sum? </p>
  
  <p>Constraints: This should be done in O(n) and in-place (without any external storage like arrays, hash-maps) (you can use extra variables/pointers)</p>
  
  <p>If this is not possible, can there be a proof given for the same?</p>