> For the complete documentation index, see [llms.txt](https://sisyphus.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://sisyphus.gitbook.io/project/leetcode-notes/linked-list/two-pointers/141.-linked-list-cycle.md).

# 141. Linked List Cycle

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

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        fast = head
        slow = head
        while(fast):
            if fast.next is None:
                return False
            fast = fast.next.next # When using linked list, always checking whether each next is valid or not.
            slow = slow.next
            if fast == slow:
                return True
        return False
```

Looping criterion is only related to the fast pointer, since it always meets the end before slow.

There are three conditions to consider:

1. There is only one node in linked list
2. There are more than one node and the fast node reaches the final loop:
   * fast jumps two steps each time, it could be the tail node
   * Or it could be the tail.next (None) 

Note: **When using linked list, always checking whether each next is valid or not.**