Swift 环形链表- LeetCode

/**
 * 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 || head?.next == nil) {
      return false          
    }
    var slow = head?
    var fast = head?.next
    while(slow != fast) {
      if (fast ==  nil || fast.next == nil) {
         return false        
      }
      slow = slow.next
      fast = fast.next.next
    }
    return true
  }
}