LinkedListCycle

// Source: LeetCode-141: https://leetcode.com/problems/linked-list-cycle/description/
// Solution Ref: https://www.youtube.com/watch?v=agkyC-rbgKM&ab_channel=FisherCoder
// Example head = [3,2,0,-4], pos = 1
// Output: true

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public var val: Int
 *     public var next: ListNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.next = nil
 *     }
 * }
 */

class Solution {
    func hasCycle(_ head: ListNode?) -> Bool {

        guard let head = head else { return false }

        var fast = head.next
        var slow = head

        while(fast != nil && slow != nil) {

            if fast === slow { return true }

            fast = fast?.next?.next
            slow = slow.next!
        }

        return false
    }
}