title: Find all pair of nos that sum up to S
body: <p>Given an array, find all pair of nos that sum up to a given value.
  There is the classic O(n) algorithm of keeping 2 pointers at the front and back and bringing them closer to find the pair. This only leads to 1 pair. What if you want all pairs.
  Bonus: Find the minimum distance pair.</p>
  
  <p>Can you do this in O(n).</p>