还是用两个指针，慢指针每次走一步，快指针每次走两步。若链表带环，两个指针必定会相遇。

 

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
        slow = fast = head
        while fast.next and fast.next.next:
            slow = slow.next
            fast = fast.next.next
            if slow==fast:
                return True
        return False