> ## Documentation Index
> Fetch the complete documentation index at: https://leetcode-py.wisl.dev/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> leetcode-py is a Python LeetCode practice environment generator with one CLI: lcpy. It is not a service or platform.
> Each problem is a directory under leetcode/ with README.md, solution.py, test_solution.py, helpers.py, and playground.ipynb. lcpy gen creates them from JSON templates bundled with the package.
> Examples are backed by tests; copy them verbatim.

# Linked List Cycle Python Solution with Tests

> Tested Python solution for LeetCode 141 with 15 pytest cases. Generate a practice environment with lcpy.

LeetCode 141, Easy. Topics: Hash Table, Linked List, Two Pointers. [View on LeetCode](https://leetcode.com/problems/linked-list-cycle/description/).

Generate this problem as a practice environment: tested reference solution, 15 [parametrized pytest cases](/practice/testing), and a playground notebook:

```bash theme={"theme":{"light":"github-light","dark":"github-dark"}}
lcpy gen -n 141   # by problem number
lcpy gen -s linked_list_cycle   # by problem name
```

## Problem

Given `head`, the head of a linked list, determine if the linked list has a cycle in it.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the `next` pointer. Internally, `pos` is used to denote the index of the node that tail's `next` pointer is connected to. **Note that `pos` is not passed as a parameter**.

Return `true` *if there is a cycle in the linked list*. Otherwise, return `false`.

### Examples

![Example 1](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist.png)

```
Input: head = [3,2,0,-4], pos = 1
Output: true
```

**Explanation:** There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).

![Example 2](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test2.png)

```
Input: head = [1,2], pos = 0
Output: true
```

**Explanation:** There is a cycle in the linked list, where the tail connects to the 0th node.

![Example 3](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test3.png)

```
Input: head = [1], pos = -1
Output: false
```

**Explanation:** There is no cycle in the linked list.

### Constraints

* The number of the nodes in the list is in the range `[0, 10^4]`.
* `-10^5 <= Node.val <= 10^5`
* `pos` is `-1` or a **valid index** in the linked-list.

**Follow up:** Can you solve it using `O(1)` (i.e. constant) memory?

## Solution

Reference implementation from [solution.py on GitHub](https://github.com/wislertt/leetcode-py/blob/main/leetcode/linked_list_cycle/solution.py), full suite in [test\_solution.py](https://github.com/wislertt/leetcode-py/blob/main/leetcode/linked_list_cycle/test_solution.py):

```python theme={"theme":{"light":"github-light","dark":"github-dark"}}
from leetcode_py import ListNode


class Solution:
    # Time: O(n)
    # Space: O(1)
    def has_cycle(self, head: ListNode[int] | None) -> bool:
        fast = head
        slow = head

        while fast and fast.next:
            assert slow is not None
            fast = fast.next.next
            slow = slow.next
            if fast is slow:
                return True

        return False
```

## Complexity

| Time | Space |
| ---- | ----- |
| O(n) | O(1)  |

## Tags

[Grind 75](/catalog/grind-75), [Grind](/catalog/grind), [Blind 75](/catalog/blind-75), [NeetCode 150](/catalog/neetcode-150), [NeetCode 250](/catalog/neetcode-250), [NeetCode All](/catalog/neetcode).