feat: Add solution For LeetCode problem 230

Author: WhiteHyun

kth-smallest-element-in-a-bst/WhiteHyun.swift

@@ -0,0 +1,34 @@
+//  
+//  230. Kth Smallest Element in a BST
+//  https://leetcode.com/problems/kth-smallest-element-in-a-bst/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/06/27.
+//  
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     public var val: Int
+ *     public var left: TreeNode?
+ *     public var right: TreeNode?
+ *     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: TreeNode?, _ right: TreeNode?) {
+ *         self.val = val
+ *         self.left = left
+ *         self.right = right
+ *     }
+ * }
+ */
+class Solution {
+  private var triedCount: Int = 0
+  func kthSmallest(_ node: TreeNode?, _ k: Int) -> Int {
+    guard let node else { return -1 }
+    let number = kthSmallest(node.left, k)
+    if number != -1 { return number }
+    if triedCount + 1 == k { return node.val }
+    triedCount += 1
+    return kthSmallest(node.right, k)
+  }
+}