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).

2 Thoughts to “LeetCode 141. Linked List Cycle”

  1. /** Java Solution **/

    /**
    * Definition for singly-linked list.
    * class ListNode {
    * int val;
    * ListNode next;
    * ListNode(int x) {
    * val = x;
    * next = null;
    * }
    * }
    */
    public class Solution {
    public boolean hasCycle(ListNode head) {

    if(head==null||head.next==null||head.next.next==null)
    return false;

    ListNode fast=head;
    ListNode slow=head;

    while(fast!=null && fast.next!=null){
    fast=fast.next.next;
    slow=slow.next;

    if(fast==slow)
    return true;

    }
    return false;
    }
    }

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