title: What is the fastest way of getting the list of lists containing a certain integer in a list of list of integers
body: <p>for example I have a list of list of integers like</p>
  <pre><code>x = [[1,2,3,4], [4,5,6], [2,3,1,9]] 
  </code></pre>
  <p>Assume the length of x is in million. In that case iterating trough each element will be very slow.
  Is there any faster way?</p>