141-Linked-List-Cycle.swift

/**
 * 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 {
        if head === nil {
            return false
        }
        
        var slow = head, fast = head
        
        while fast != nil && fast?.next != nil {
            slow = slow?.next
            fast = fast?.next?.next
            
            if fast === slow {
                return true
            }
        }
        return false
    }
}