> For the complete documentation index, see [llms.txt](https://zcjian.gitbook.io/project/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://zcjian.gitbook.io/project/linked-list/linked-list-cycle-ii.md).

# Linked List Cycle II

LeetCode 142

```python
# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def detectCycle(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        slow = fast = head
        
        while fast and fast.next and fast.next.next:
            slow = slow.next
            fast = fast.next.next
            # 利用 is 
            if slow is fast:
                slow = head
                while slow is not fast:
                    slow = slow.next
                    fast = fast.next
                return slow
        return 
```