[Tree] Add a solution to Binary Tree Upside Down

Author: Yi Gu

Tree/BinaryTreeUpsideDown.swift

@@ -0,0 +1,37 @@
+/**
+ * Question Link: https://leetcode.com/problems/binary-tree-upside-down/
+ * Primary idea: Mark left one as current node, change its left and right and 
+ *               keep going to right until to the leaf
+ * Time Complexity: O(n), Space Complexity: O(1)
+ *
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     public var val: Int
+ *     public var left: TreeNode?
+ *     public var right: TreeNode?
+ *     public init(_ val: Int) {
+ *         self.val = val
+ *         self.left = nil
+ *         self.right = nil
+ *     }
+ * }
+ */
+
+class BinaryTreeUpsideDown {
+    func upsideDownBinaryTree(_ root: TreeNode?) -> TreeNode? {
+        var parent: TreeNode?
+        var node: TreeNode? = root
+        var right: TreeNode?
+        
+        while node != nil {
+            let left = node!.left
+            node!.left = right
+            right = node!.right
+            node!.right = parent
+            parent = node
+            node = left
+        }
+        
+        return parent
+    }
+}