题解 | #判断链表中是否有环#

# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

#
# 
# @param head ListNode类 
# @return bool布尔型
#
class Solution:
    def hasCycle(self , head: ListNode) -> bool:
        slow = head 
        fast = head 
        while fast and fast.next:
            slow = slow.next 
            fast = fast.next.next
            if slow==fast:
                return True 
        return False

双指针法：一个快指针和一个快指针在同一个链表中移动时，如果它们相遇则代表有环。