142. 环形链表 II - 力扣（LeetCode）

# 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]:
        if not head or not head.next:
            return None

        # 初始化快慢指针
        slow, fast = head, head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
            if slow == fast:  # 检测到环
                break
        else:
            return None  # 无环

        # 找到环的入口
        ptr1, ptr2 = head, slow
        while ptr1 != ptr2:
            ptr1 = ptr1.next
            ptr2 = ptr2.next

        return ptr1  # 环的入口节点