title: Verify if an undirected graph has cycles
body: <p>I know that this question has been asked some times. But I just can't understand yet and those questions are too old to reply...   </p>
  
  <p>I Read about <a href="https://stackoverflow.com/q/30850648/2054434">Kahn's Algorithm</a><br>
  Also about <a href="https://stackoverflow.com/a/526371/2054434">DFS</a><br>
  Plus <a href="https://stackoverflow.com/a/261595/2054434">Tarjan's strongly connected components algorithm</a><br>
  But i Just can't understand the process to make it work.<br>
  All I've done so far is to initialized and feed the graph.<br>
  And found the shortest path from vertéx A to B.</p>