title: Good way to store data about u customers who visited w webpages on d days
body: <p>This is interview question from Amazon.</p>
  
  <p>Given <code>u</code> lists of customers who visited <code>w</code> webpages on <code>d</code> days, design a data structure to get customers who have visited the website on exactly <code>k</code> days and should have visited at least <code>m</code> distinct pages altogether.</p>
  
  <p>Assume <code>u</code>, <code>w</code> and <code>d</code> are large numbers.</p>
  
  <p>How do we store this information? Maybe we can use hash maps, trees, etc.</p>