# LeetCode 141. Linked List Cycle

## Description

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

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

class Solution:
def hasCycle(self, head: ListNode) -> bool:

visited = set()

else:
return True

return False

• Time complexity: O(N).
• Space complexity: O(N).

## 3 Thoughts to “LeetCode 141. Linked List Cycle”

1. /** Java Solution **/

/**
* class ListNode {
* int val;
* ListNode next;
* ListNode(int x) {
* val = x;
* next = null;
* }
* }
*/
public class Solution {

return false;

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

if(fast==slow)
return true;

}
return false;
}
}