# [Go] LeetCode 141. Linked List Cycle

Jan 20 · 2 min read

# 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.

Follow up:

Can you solve it using O(1) (i.e. constant) memory?

# Solution

/*

Approach 1: Hash TableTime complexity : O(n)

Space complexity: O(n)

*/

type ListNode struct {

Val int

Next *ListNode

}type NodesSeen []*ListNodefunc hasCycle1(head *ListNode) bool {

nodesSeen := NodesSeen{}

for head != nil{

if nodesSeen.contain(head) {

return true

}

nodesSeen = append(nodesSeen,head)

head = head.Next

}

return false

}func(n NodesSeen) contain(head *ListNode) bool{

for _,val := range n{

if *val == *head{

return true

}

}

return false

}/*

Approach 2: Two Pointerstime complexity is O(N+K), which is O(n)

Space complexity : O(1).

*/

func hasCycle2(head *ListNode) bool {

if head == nil {

return false

}

slow := head

fast := head.Next

//Check whether the fast runner will eventually meet the slow runner or not

//if not, head has no cycle

for slow != nil && fast != nil && fast.Next != nil{

if *slow == *fast {

return true

}

slow = slow.Next

fast = fast.Next.Next

}

return false

}