力扣204

# Definition for singly-linked list.

# class ListNode:

# def __init__(self, x):

# self.val = x

# self.next = None

class Solution:

def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:

# 初始化快慢指针，起点都是head

slow = fast = head

# 第一步：快慢指针遍历，找环内相遇点

while fast and fast.next:

slow = slow.next # 慢指针每次走1步

fast = fast.next.next # 快指针每次走2步

# 相遇 → 有环

if slow == fast:

# 第二步：从头节点出发，与相遇点同步走，相遇即环入口

p = head

while p != slow:

p = p.next

slow = slow.next

return p # 返回入口节点

# 快指针到尾 → 无环

return None