Linked List Cycle

April 19, 2019

Introduction

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

Simple solution:

type ListNode struct {
    Val int
    Next *ListNode
}

func hasCycle(head *ListNode) bool {

    if head == nil {
        return false
    }

    for i, j := one(head), two(head); i != nil && j != nil; i, j = one(i), two(j) {
       if i == j {
            return true
       }
    }

    return false
}

func one(node *ListNode) *ListNode {
    return node.Next
}

func two(node *ListNode) *ListNode {
    next := node.Next
    if next == nil {
        return nil
    } else {
        return next.Next
    }
}

Explanation

We need two pointers, i will do one step a time, j will do two steps a time. We have cycle only if they meet each other.