Problem Statement

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.

Example 1:

flowchart LR a((3))-->b((2)) b-->c((0)) c-->d(("-4")) d-->b
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:

flowchart LR a((1))-->b((2)) b-->a
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:

flowchart LR a((1))
Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

Solution

Python Implementation

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

class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        """
            T: O(N)
            S: O(N)
        """
        mp = set()
        curr = head

        while curr and curr.next:
            if curr in mp:
                return True
            else:
                mp.add(curr)
            
            curr = curr.next

        return False

Leetcode 141: Linked List Cycle