LeetCode 141. Linked List Cycle

Description

https://leetcode.com/problems/palindrome-linked-list/

Given a linked list, determine if it has a cycle in it.

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.

Example 2:

Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.

Example 3:

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

Explanation

If any of the node has been visited, the linked list has a cycle.

Python Solution

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

class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        
        visited = set()
        
        while head != None:
            if head not in visited:
                visited.add(head)
            else:
                return True
        
            head = head.next
            
        return False
      
  • Time complexity: O(N).
  • Space complexity: O(N).

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