[Swift]LeetCode142. 環形鏈表 II | Linked List Cycle II

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Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

Note: Do not modify the linked list.

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

---

給定一個鏈表，返回鏈表開始入環的第一個節點。 若是鏈表無環，則返回 null。

說明：不容許修改給定的鏈表。

進階：
你是否能夠不用額外空間解決此題？

---

 1 import Foundation
 2 
 3 class LinkedListCycleII {
 4 
 5     public class ListNode {
 6         
 7         public var val: Int
 8         public var next: ListNode?
 9         
10         public init(_ val: Int) {
11             self.val = val
12             self.next = nil
13         }
14     }
15     
16     // Waiting to be judged.
17     func detectCycle(_ head: ListNode?) -> ListNode? {
18         var slow = head
19         var fast = head
20         var h = head
21         while fast != nil && fast?.next != nil {
22             slow = slow?.next
23             fast = fast?.next?.next
24             if slow === fast {
25                 while slow !== head {
26                     h = h?.next
27                     slow = slow?.next
28                 }
29                 return h
30             }
31         }
32         return nil
33     }
34 }