为链表篇 增加 Swift 实现

animation-simulation/链表篇/面试题 02.03. 链表中间节点.md

@@ -118,3 +118,20 @@ class Solution:
         return slow
 ```
 
+Swift Code：
+
+```swift
+class Solution {
+    func middleNode(_ head: ListNode?) -> ListNode? {
+        var fast = head //快指针
+        var slow = head //慢指针
+        //循环条件，思考一下跳出循环的情况
+        while fast != nil && fast?.next != nil {
+            fast = fast?.next?.next
+            slow = slow?.next
+        }
+        //返回slow指针指向的节点
+        return slow
+    }
+}
+```