title: Get the set of numbers in a list whose sum matches the required number given
body: <p>I am currently working on a problem -the statement is like </p>
  
  <p>I am given an list of numbers and A number-s. I need to find out the collection(s) of numbers from the list of numbers whose sum corresponds to the given number s.</p>
  
  <p>Now I can do this using nested for loops or even <code>itertools.combinations()</code> -BUT both of them have big time complexities..</p>
  
  <p>Can anyone please help me what should be my approach or what algorithm should I use to have a clean and simple code to achieve it in better time complexity?</p>