# LeetCode 141: Linked List Cycle — Step-by-Step Visual Trace

**Easy** — Linked List | Two Pointers | Hash Table

## The Problem

Given the head of a linked list, determine if the linked list has a cycle in it. A cycle exists if there is some node in the list that can be reached again by continuously following the next pointer.

## Approach

Uses Floyd's Cycle Detection Algorithm (tortoise and hare) with two pointers moving at different speeds. If there's a cycle, the fast pointer will eventually meet the slow pointer; if there's no cycle, the fast pointer will reach the end.

**Time:** O(n) · **Space:** O(1)

## Code

```python
class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        if not head or not head.next:
            return False

        slow = head
        fast = head.next

        while slow != fast:
            if not fast or not fast.next:
                return False
            slow = slow.next
            fast = fast.next.next

        return True
```

## Watch It Run

> **[Open interactive visualization](https://tracelit.dev/app?trace=0141_linked-list-cycle)**

> **Try it yourself:** Open [TraceLit](https://tracelit.dev/app?trace=0141_linked-list-cycle) and step through every line.

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