[Tree] Add a solution to Kth Smallest Element in a BST

README.md

@@ -28,7 +28,7 @@
 * [Microsoft](#microsoft)
 
 ## Progress
-[Problem Status](#problem-status) shows the latest progress to all 800+ questions. Currently we have 245 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them. Thank you for great contributions from [CharleneJiang](https://github.com/CharleneJiang), [ReadmeCritic](https://github.com/ReadmeCritic), [demonkoo](https://github.com/demonkoo), [DaiYue](https://github.com/DaiYue), [Quaggie](https://github.com/Quaggie) and [jindulys](https://github.com/jindulys).
+[Problem Status](#problem-status) shows the latest progress to all 800+ questions. Currently we have 246 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them. Thank you for great contributions from [CharleneJiang](https://github.com/CharleneJiang), [ReadmeCritic](https://github.com/ReadmeCritic), [demonkoo](https://github.com/demonkoo), [DaiYue](https://github.com/DaiYue), [Quaggie](https://github.com/Quaggie) and [jindulys](https://github.com/jindulys).
 
 
 ## Array
@@ -156,6 +156,7 @@
 [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)|
 [Binary Tree Zigzag Level Order Traversal](https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/)| [Swift](./Tree/BinaryTreeZigzagLevelOrderTraversal.swift)| Medium| O(n)| O(n)|
 [Binary Tree Vertical Order Traversal](https://leetcode.com/problems/binary-tree-vertical-order-traversal/)| [Swift](./Tree/BinaryTreeVerticalOrderTraversal.swift)| Medium| O(n)| O(n)|
+[Kth Smallest Element in a BST](https://leetcode.com/problems/kth-smallest-element-in-a-bst/)| [Swift](./Tree/KthSmallestElementBST.swift)| Medium| O(n)| O(n)|
 [Binary Tree Right Side View](https://leetcode.com/problems/binary-tree-right-side-view/)| [Swift](./Tree/BinaryTreeRightSideView.swift)| Medium| O(n)| O(n)|
 [Construct Binary Tree from Preorder and Inorder Traversal](https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/)| [Swift](./Tree/ConstructBinaryTreePreorderInorder.swift)| Medium| O(n)| O(n)|
 [Construct Binary Tree from Inorder and Postorder Traversal](https://leetcode.com/problems/construct-binary-tree-from-inorder-and-postorder-traversal/)| [Swift](./Tree/ConstructBinaryTreeInorderPostorder.swift)| Medium| O(n)| O(n)|
@@ -589,7 +590,7 @@
 |		|	233	|	[Number of Digit One](https://leetcode.com/problems/number-of-digit-one/)	|	Hard	|
 |		|	232	|	[Implement Queue using Stacks](https://leetcode.com/problems/implement-queue-using-stacks/)	| Easy	|
 |	[Swift](./Math/PowerTwo.swift)	|	231	|	[Power of Two](https://leetcode.com/problems/power-of-two/)	| Easy	|
-|		|	230	|	[Kth Smallest Element in a BST](https://leetcode.com/problems/kth-smallest-element-in-a-bst/)	|	Medium	|
+|	[Swift](./Tree/KthSmallestElementBST.swift)	|	230	|	[Kth Smallest Element in a BST](https://leetcode.com/problems/kth-smallest-element-in-a-bst/)	|	Medium	|
 |	[Swift](./Array/MajorityElementII.swift)	|	229	|	[Majority Element II](https://leetcode.com/problems/majority-element-ii/)	|	Medium	|
 |	[Swift](./Array/SummaryRanges.swift)	|	228	|	[Summary Ranges](https://leetcode.com/problems/summary-ranges/)	|	Medium	|
 |		|	227	|	[Basic Calculator II](https://leetcode.com/problems/basic-calculator-ii/)	|	Medium	|

Tree/KthSmallestElementBST.swift

@@ -0,0 +1,41 @@
+/**
+ * Question Link: https://leetcode.com/problems/kth-smallest-element-in-a-bst/
+ * Primary idea: use stack to do inorder traverse and track k to find answer
+ * Time Complexity: O(n), Space Complexity: O(n)
+ * 
+ * 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 KthSmallestElementBST {
+    func kthSmallest(_ root: TreeNode?, _ k: Int) -> Int {
+        var stack = [TreeNode](), currentNode = root, k = k
+
+        while !stack.isEmpty || currentNode != nil {
+            if currentNode != nil {
+                stack.append(currentNode!)
+                currentNode = currentNode!.left
+            } else {
+                let node = stack.removeLast()
+                k -= 1
+
+                if k == 0 {
+                    return node.val
+                }
+
+                currentNode = node.right
+            }
+        }
+
+        return -1
+    }
+}