title: Check if two linked lists merge. If so, where?
body: <p>This question may be old, but I couldn't think of an answer.</p>
  
  <p>Say, there are two lists of different lengths, <strong>merging at a point</strong>; how do we know where the merging point is?</p>
  
  <p>Conditions:</p>
  
  <ol>
  <li>We don't know the length </li>
  <li>We should parse each list only once.</li>
  </ol>
  
  <p><img src="https://i.stack.imgur.com/xjn0S.png" alt="Example of two merged linked lists."></p>