题解 | #判断链表中是否有环# -双指针

# 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:
            return False

        f = head #快指针
        s = head #慢指针

        while f and s:
            s = s.next
            if f.next:
                f = f.next.next
            else:
                return False
            
            if s == f:
                return True
        
        return False