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?

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        if head is None or head.next is None:
            return False
        slow = head
        fast = head.next
        while fast != slow:
            if fast is None or fast.next is None:
                return False
            slow = slow.next
            fast = fast.next.next
        return True

No extra space solution：

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        while head:
            if head.val is False:
                return True
            head.val = False
            head = head.next
        return False