feat: add swift implementation to lcof2 problem: No.052 (#3067)

Author: klever34

lcof2/剑指 Offer II 052. 展平二叉搜索树/README.md

@@ -326,6 +326,58 @@ struct TreeNode* increasingBST(struct TreeNode* root) {
 }
 ```
 
+#### Swift
+
+```swift
+/* class TreeNode {
+*     var val: Int
+*     var left: TreeNode?
+*     var right: TreeNode?
+*     init() {
+*         self.val = 0
+*         self.left = nil
+*         self.right = nil
+*     }
+*     init(_ val: Int) {
+*         self.val = val
+*         self.left = nil
+*         self.right = nil
+*     }
+*     init(_ val: Int, _ left: TreeNode?, _ right: TreeNode?) {
+*         self.val = val
+*         self.left = left
+*         self.right = right
+*     }
+* }
+*/
+
+class Solution {
+    func increasingBST(_ root: TreeNode?) -> TreeNode? {
+        var head: TreeNode? = nil
+        var tail: TreeNode? = nil
+        var stack = [TreeNode]()
+        var cur = root
+        
+        while !stack.isEmpty || cur != nil {
+            while cur != nil {
+                stack.append(cur!)
+                cur = cur?.left
+            }
+            cur = stack.removeLast()
+            if head == nil {
+                head = cur
+            } else {
+                tail?.right = cur
+            }
+            tail = cur
+            cur?.left = nil
+            cur = cur?.right
+        }
+        return head
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof2/剑指 Offer II 052. 展平二叉搜索树/Solution.swift

@@ -0,0 +1,47 @@
+/* class TreeNode {
+*     var val: Int
+*     var left: TreeNode?
+*     var right: TreeNode?
+*     init() {
+*         self.val = 0
+*         self.left = nil
+*         self.right = nil
+*     }
+*     init(_ val: Int) {
+*         self.val = val
+*         self.left = nil
+*         self.right = nil
+*     }
+*     init(_ val: Int, _ left: TreeNode?, _ right: TreeNode?) {
+*         self.val = val
+*         self.left = left
+*         self.right = right
+*     }
+* }
+*/
+
+class Solution {
+    func increasingBST(_ root: TreeNode?) -> TreeNode? {
+        var head: TreeNode? = nil
+        var tail: TreeNode? = nil
+        var stack = [TreeNode]()
+        var cur = root
+        
+        while !stack.isEmpty || cur != nil {
+            while cur != nil {
+                stack.append(cur!)
+                cur = cur?.left
+            }
+            cur = stack.removeLast()
+            if head == nil {
+                head = cur
+            } else {
+                tail?.right = cur
+            }
+            tail = cur
+            cur?.left = nil
+            cur = cur?.right
+        }
+        return head
+    }
+}