具体题目与思路见：https://programmercarl.com/0142.%E7%8E%AF%E5%BD%A2%E9%93%BE%E8%A1%A8II.html#%E6%80%9D%E8%B7%AF
该文章详细介绍了如何寻找环入口点，以及数学原理。
class ListNode(object):def __init__(self, x):self.val = xself.next = Noneclass Solution(object):def detectCycle(self, head):""":type head: ListNode:rtype: ListNode"""# fast, slow = head # 快慢指针初始化为 头节点fast = ListNode()slow = ListNode()fast = headslow = headwhile fast and fast.next: # 是 fast and fast.nextfast = fast.next.nextslow = slow.next# 当快慢指针相遇if fast == slow: # 相遇就要开始找入环点 为什么相遇？说明一定有环 所以这里第一步可以用来判断是否有环index1 = fastindex2 = headwhile index1 != index2:index1 = index1.nextindex2 = index2.nextreturn index1return None
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