[Tree] Add a solution to Convert Sorted Array to Binary Search Tree

Author: soapyigu

Diff for: README.md

@@ -4,7 +4,7 @@
 ![Leetcode](./logo.png?style=centerme)
 
 ## Progress
-[Problem Status](#problem-status) shows the latest progress to all 1000+ questions. Currently we have 278 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them.
+[Problem Status](#problem-status) shows the latest progress to all 1000+ questions. Currently we have 279 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them.
 
 ## Contributors
 
@@ -173,6 +173,7 @@
 [Balanced Binary Tree](https://leetcode.com/problems/balanced-binary-tree/)| [Swift](./Tree/BalancedBinaryTree.swift)| Easy|  O(n)| O(n)|
 [Sum of Left Leaves](https://leetcode.com/problems/sum-of-left-leaves/)| [Swift](./Tree/SumLeftLeaves.swift)| Easy|  O(n)| O(1)|
 [Flatten Binary Tree to Linked List](https://leetcode.com/problems/flatten-binary-tree-to-linked-list/)| [Swift](./Tree/FlattenBinaryTreeLinkedList.swift)| Medium| O(n)| O(1)|
+[Convert Sorted Array to Binary Search Tree](https://leetcode.com/problems/convert-sorted-array-to-binary-search-tree/)| [Swift](./Tree/ConvertSortedArrayBinarySearchTree.swift)| Easy| O(n)| O(1)|
 [Validate Binary Search Tree](https://leetcode.com/problems/validate-binary-search-tree/)| [Swift](./Tree/ValidateBinarySearchTree.swift)| Medium| O(n)| O(log n)|
 [Binary Tree Level Order Traversal](https://leetcode.com/problems/binary-tree-level-order-traversal/)| [Swift](./Tree/BinaryTreeLevelOrderTraversal.swift)| Easy| O(n)| O(n)|
 [Binary Tree Level Order Traversal II](https://leetcode.com/problems/binary-tree-level-order-traversal-ii/)| [Swift](./Tree/BinaryTreeLevelOrderTraversalII.swift)| Easy| O(n)| O(n)|
@@ -741,7 +742,7 @@
 |	[Swift](./Tree/MinimumDepthOfBinaryTree.swift)	|	111	|	[Minimum Depth of Binary Tree](https://oj.leetcode.com/problems/minimum-depth-of-binary-tree/)	| Easy	|
 |	[Swift](./Tree/BalancedBinaryTree.swift)	|	110	|	[Balanced Binary Tree](https://oj.leetcode.com/problems/balanced-binary-tree/)	| Easy	|
 |		|	109	|	[Convert Sorted List to Binary Search Tree](https://oj.leetcode.com/problems/convert-sorted-list-to-binary-search-tree/)	| Medium	|
-|		|	108	|	[Convert Sorted Array to Binary Search Tree](https://oj.leetcode.com/problems/convert-sorted-array-to-binary-search-tree/)	| Medium	|
+|	[Swift](./Tree/ConvertSortedArrayBinarySearchTree.swift)	|	108	|	[Convert Sorted Array to Binary Search Tree](https://oj.leetcode.com/problems/convert-sorted-array-to-binary-search-tree/)	| Medium	|
 |	[Swift](./Tree/BinaryTreeLevelOrderTraversalII.swift)	|	107	|	[Binary Tree Level Order Traversal II](https://oj.leetcode.com/problems/binary-tree-level-order-traversal-ii/)	| Easy	|
 |	[Swift](./Tree/ConstructBinaryTreeInorderPostorder.swift)	|	106	|	[Construct Binary Tree from Inorder and Postorder Traversal](https://oj.leetcode.com/problems/construct-binary-tree-from-inorder-and-postorder-traversal/)	| Medium	|
 |	[Swift](./Tree/ConstructBinaryTreePreorderInorder.swift)	|	105	|	[Construct Binary Tree from Preorder and Inorder Traversal](https://oj.leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/)	| Medium	|

Diff for: Tree/ConvertSortedArrayBinarySearchTree.swift

@@ -0,0 +1,37 @@
+/**
+ * Question Link: https://leetcode.com/problems/convert-sorted-array-to-binary-search-tree/
+ * Primary idea: recursion, the root of subtree should always be mid point of the subarray
+ * 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 ConvertSortedArrayBinarySearchTree {
+    func sortedArrayToBST(_ nums: [Int]) -> TreeNode? {
+        return sortedArrayToBST(nums, 0, nums.count - 1)
+    }
+    
+    private func sortedArrayToBST(_ nums: [Int], _ leftIdx: Int, _ rightIdx: Int) -> TreeNode? {
+        guard leftIdx <= rightIdx else {
+            return nil
+        }
+        
+        let mid = (rightIdx - leftIdx) / 2 + leftIdx
+        let root = TreeNode(nums[mid])
+        
+        root.left = sortedArrayToBST(nums, leftIdx, mid - 1)
+        root.right = sortedArrayToBST(nums, mid + 1, rightIdx)
+        
+        return root
+    }
+}