141. Linked List Cycle

Problem

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

Follow up:
Can you solve it without using extra space?

Problem Summary

Determine whether a linked list has a cycle, without using extra space.

Solution

Use 2 pointers, where one pointer is ahead of the other. The fast pointer moves 2 steps at a time, and the slow pointer moves 1 step at a time. If a cycle exists, then the fast pointer will eventually catch up with the slow pointer after some number of loops.

Code

package leetcode

import (
	"github.com/halfrost/leetcode-go/structures"
)

// ListNode define
type ListNode = structures.ListNode

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */

func hasCycle(head *ListNode) bool {
	fast := head
	slow := head
	for fast != nil && fast.Next != nil {
		fast = fast.Next.Next
		slow = slow.Next
		if fast == slow {
			return true
		}
	}
	return false
}