[LinkedList] optimize Solution for Reorder List

LinkedList/ReorderList.swift

@@ -16,52 +16,57 @@
 
 class ReorderList {
     func reorderList(head: ListNode?) {
-        if head == nil || head!.next == nil {
+        guard let head = head else {
             return
         }
-
-        // split the list
-        var prev = head
-        var post = head!.next
-
+
+        var prev: ListNode? = head
+        var post: ListNode? = head
+
+        _split(&prev, &post)
+        prev = head
+
+        post = _reverse(&post)
+
+        _merge(&prev, &post)
+    }
+
+    private func _split(inout prev: ListNode?, inout _ post: ListNode?) {
         while post != nil && post!.next != nil {
             prev = prev!.next
             post = post!.next!.next
         }
-        
+
         post = prev!.next
         prev!.next = nil
-        prev = head
-
-        // reverse the second list
-        post = _reverse(post)
-
-        // merge lists
-        while prev != nil && post != nil{
-            let preNext = prev!.next
-            let posNext = post!.next
-
-            prev!.next = post
-            post!.next = preNext
-
-            prev = preNext
-            post = posNext
-        }
     }
-    
-    private func _reverse(head: ListNode?) -> ListNode?{
+
+    private func _reverse(inout head: ListNode?) -> ListNode? {
         var prev = head
         var temp: ListNode?
-        
+
         while prev != nil {
             let post = prev!.next
-            
+
             prev!.next = temp
-            
+
             temp = prev
             prev = post
         }
-        
+
         return temp
     }
+
+    private func _merge(inout prev: ListNode?, inout _ post: ListNode?) {
+        while prev != nil && post != nil{
+            let preNext = prev!.next
+            let posNext = post!.next
+
+            prev!.next = post
+            post!.next = preNext
+
+            prev = preNext
+            post = posNext
+        }
+    }
 }