feat: add swift implementation to lcof2 problem: No.028 (#3027)

Author: klever34

lcof2/剑指 Offer II 028. 展平多级双向链表/README.md

@@ -244,6 +244,54 @@ public:
 };
 ```
 
+#### Swift
+
+```swift
+/* class Node {
+*     var val: Int
+*     var prev: Node?
+*     var next: Node?
+*     var child: Node?
+
+*     init(_ val: Int) {
+*         self.val = val
+*         self.prev = nil
+*         self.next = nil
+*         self.child = nil
+*     }
+* }
+*/
+
+class Solution {
+    private var dummy = Node(0)
+    private var tail: Node?
+
+    func flatten(_ head: Node?) -> Node? {
+        guard let head = head else {
+            return nil
+        }
+        tail = dummy
+        preOrder(head)
+        dummy.next?.prev = nil
+        return dummy.next
+    }
+
+    private func preOrder(_ node: Node?) {
+        guard let node = node else {
+            return
+        }
+        let next = node.next
+        let child = node.child
+        tail?.next = node
+        node.prev = tail
+        tail = node
+        node.child = nil
+        preOrder(child)
+        preOrder(next)
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof2/剑指 Offer II 028. 展平多级双向链表/Solution.swift

@@ -0,0 +1,43 @@
+/* class Node {
+*     var val: Int
+*     var prev: Node?
+*     var next: Node?
+*     var child: Node?
+
+*     init(_ val: Int) {
+*         self.val = val
+*         self.prev = nil
+*         self.next = nil
+*         self.child = nil
+*     }
+* }
+*/
+
+class Solution {
+    private var dummy = Node(0)
+    private var tail: Node?
+
+    func flatten(_ head: Node?) -> Node? {
+        guard let head = head else {
+            return nil
+        }
+        tail = dummy
+        preOrder(head)
+        dummy.next?.prev = nil
+        return dummy.next
+    }
+
+    private func preOrder(_ node: Node?) {
+        guard let node = node else {
+            return
+        }
+        let next = node.next
+        let child = node.child
+        tail?.next = node
+        node.prev = tail
+        tail = node
+        node.child = nil
+        preOrder(child)
+        preOrder(next)
+    }
+}