title: Find longest repetitive sequence in a string
body: <p>I need to find the longest sequence in a string with the caveat that the sequence must be repeated three or more times. So, for example, if my string is:</p>
  
  <p><strong>fdwaw4helloworldvcdv1c3xcv3xcz1sda21f2sd1ahelloworldgafgfa4564534321fadghelloworld</strong></p>
  
  <p>then I would like the value "<strong>helloworld</strong>" to be returned. </p>
  
  <p>I know of a few ways of accomplishing this but the problem I'm facing is that the actual string is absurdly large so I'm really looking for a method that can do it in a timely fashion. </p>