title: Get previous/next item of a given item in a List<>
body: <p>Says I have this List : 1, 3, 5, 7, 9, 13 </p>
  
  <p>For example, given value is : 9, the previous item is 7 and the next item is 13 </p>
  
  <p>How can I achieve this using C#? </p>