141. Linked List Cycle

Updated: 2024-03-12
2 min read

LeetCode problem 141

Problem Statement

The problem asks us to determine if a given linked list contains a cycle. A cycle in a linked list occurs when a node’s next pointer points back to a previous node in the list, causing an infinite loop.

Hints & Tips

In this problem, you can take advantage of the Floyd’s “Tortoise and Hare” cycle detection algorithm. This algorithm allows you to detect a cycle in O(1) space and O(n) time complexity, where n is the number of nodes.


  1. Use two pointers, slow and fast. Initially, point them to the head of the linked list.
  2. Move the slow pointer one step at a time, and the fast pointer two steps at a time.
  3. If there’s a cycle, the fast pointer will eventually catch up to the slow pointer. If not, the fast pointer will reach the end of the list (None).
  • Step 1: Initialize slow = head and fast = head.
  • Step 2: Move slow one step and fast two steps in a loop.
  • Step 3: If fast and slow meet at any point, return True. If fast reaches the end, return False.

Solution | Pointers

Here’s the Python code for this algorithm, commented for clarity:

class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None

def hasCycle(head: ListNode) -> bool:
    # Initialize slow and fast pointers to head
    slow = head
    fast = head
    while fast and fast.next:
        slow = slow.next        # Move slow one step
        fast = fast.next.next   # Move fast two steps
        if slow == fast:
            return True

    return False

Solution | Visited

class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        visited = set()
        cur = head
        while cur:
            if cur.next in visited:
                return True
            cur = cur.next
        return False