feat: add swift implementation to lcof problem: No.36 (doocs#2906)

Author: klever34

lcof/面试题36. 二叉搜索树与双向链表/README.md

@@ -344,6 +344,66 @@ public class Solution {
 }
 ```
 
+#### Swift
+
+```swift
+/* Definition for a Node.
+* public class Node {
+*     public var val: Int
+*     public var left: Node?
+*     public var right: Node?
+
+*     public init() {
+*         self.val = 0
+*         self.left = nil
+*         self.right = nil
+*     }
+
+*     public init(_ val: Int) {
+*         self.val = val
+*         self.left = nil
+*         self.right = nil
+*     }
+
+*     public init(_ val: Int, _ left: Node?, _ right: Node?) {
+*         self.val = val
+*         self.left = left
+*         self.right = right
+*     }
+* }
+*/
+
+class Solution {
+    private var head: Node?
+    private var pre: Node?
+
+    func treeToDoublyList(_ root: Node?) -> Node? {
+        if root == nil {
+            return nil
+        }
+        dfs(root)
+        head?.left = pre
+        pre?.right = head
+        return head
+    }
+
+    private func dfs(_ root: Node?) {
+        guard let root = root else {
+            return
+        }
+        dfs(root.left)
+        if let preNode = pre {
+            preNode.right = root
+        } else {
+            head = root
+        }
+        root.left = pre
+        pre = root
+        dfs(root.right)
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof/面试题36. 二叉搜索树与双向链表/Solution.swift

@@ -0,0 +1,55 @@
+/* Definition for a Node.
+* public class Node {
+*     public var val: Int
+*     public var left: Node?
+*     public var right: Node?
+
+*     public init() {
+*         self.val = 0
+*         self.left = nil
+*         self.right = nil
+*     }
+
+*     public init(_ val: Int) {
+*         self.val = val
+*         self.left = nil
+*         self.right = nil
+*     }
+
+*     public init(_ val: Int, _ left: Node?, _ right: Node?) {
+*         self.val = val
+*         self.left = left
+*         self.right = right
+*     }
+* }
+*/
+
+class Solution {
+    private var head: Node?
+    private var pre: Node?
+
+    func treeToDoublyList(_ root: Node?) -> Node? {
+        if root == nil {
+            return nil
+        }
+        dfs(root)
+        head?.left = pre
+        pre?.right = head
+        return head
+    }
+
+    private func dfs(_ root: Node?) {
+        guard let root = root else {
+            return
+        }
+        dfs(root.left)
+        if let preNode = pre {
+            preNode.right = root
+        } else {
+            head = root
+        }
+        root.left = pre
+        pre = root
+        dfs(root.right)
+    }
+}