为链表篇 增加 Swift 实现

animation-simulation/链表篇/leetcode141环形链表.md

@@ -108,3 +108,20 @@ class Solution:
         return False
 ```
 
+Swift Code：
+
+```swift
+class Solution {
+    func hasCycle(_ head: ListNode?) -> Bool {
+        var fast = head, slow = head
+        while fast != nil && fast?.next != nil {
+            fast = fast?.next?.next
+            slow = slow?.next
+            if fast === slow {
+                return true
+            }
+        }
+        return false
+    }
+}
+```