141. 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?

Python

 
# 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 is None:
            return False
        node1 = head
        node2 = head
        while True:
            if node1.next is not None:
                node1 = node1.next.next
                node2 = node2.next
                if node1 is None or node2 is None:
                    return False
                elif node1 == node2:
                    return True
            else:
                return False