Linked List Cycle

Given a linked list, determine if it has a cycle in it.

Follow up: Can you solve it without using extra space?

URL: https://leetcode.com/problems/linked-list-cycle/

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        if head == None:
            return False
        else:
            fast = head
            slow = head

            while fast != None and fast.next != None:
                slow = slow.next
                fast = fast.next.next
                if fast == slow:
                    break

            if fast == None or fast.next == None:
                return False
            elif fast == slow:
                return True

            return False