title: Getting top 100 URL from a log file
body: <p>One of my friends was asked the following question in an interview. Can anyone tell me how to solve it?</p>
  
  <p>We have a fairly large log file, about 5GB. Each line of the log file contains an url which a user has visited on our site. We want to figure out what's the most popular 100 urls visited by our users. How to do it?</p>