title: Retrieving all the pairs of numbers that sum to a target in a list
body: <p>Suppose I have a list <code>[1,2,3,4,4]</code> and a target <code>5</code>. I need to find all the pairs from the list which sum to 5, i.e., <code>(1,4),(1,4),(2,3)</code>. Can someone suggest me an algorithm how to solve it in less than <code>O(n^2)</code>.
  I cam across this question while preparing for interview but I am not able to solve it in less than <code>O(n^2)</code>.
     Any Help is appreciated</p>