Hi, here's your problem today. This problem was recently asked by Uber:

Given a linked list, determine if the linked list has a cycle in it. For notation purposes, we use an integer pos which represents the zero-indexed position where the tail connects to.

Example:
Input: head = [4,3,2,1,0], pos = 1.  
Output: true
The example indicates a list where the tail connects to the second node.

<pre lang=py>
class ListNode(object):
  def __init__(self, x):
    self.val = x
    self.next = None

class Solution(object):
  def hasCycle(self, head):
    # Fill this in.

testHead = ListNode(4)
node1 = ListNode(3)
testHead.next = node1
node2 = ListNode(2)
node1.next = node2
node3 = ListNode(1)
node2.next = node3
testTail = ListNode(0)
node3.next = testTail
testTail.next = node1

print(Solution().hasCycle(testHead))
# True
</pre>