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:
        if not head:#如果是空链表
            return False
        
        fast, slow = head, head#快、慢指针
        while slow and fast:
            slow = slow.next#慢指针每次前进一步
            if fast.next:#快指针每次前进两步（若允许）
                fast = fast.next.next
            else:#如果快指针到达了链表末尾，无环
                return False
            if slow==fast:#快慢指针相遇，有环
                return True
        return False#如果快指针到达了链表末尾，无环