feat: Add solution for LeetCode problem 235

Author: WhiteHyun

lowest-common-ancestor-of-a-binary-search-tree/WhiteHyun.swift

@@ -0,0 +1,35 @@
+//  
+//  235. Lowest Common Ancestor of a Binary Search Tree
+//  https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/06/12.
+//  
+
+/**
+ * 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 Solution {
+  func lowestCommonAncestor(_ root: TreeNode?, _ p: TreeNode?, _ q: TreeNode?) -> TreeNode? {
+    if root == nil { return nil }
+    if root == p { return p }
+    if root == q { return q }
+    let left = lowestCommonAncestor(root?.left, p, q)
+    let right = lowestCommonAncestor(root?.right, p, q)
+
+    if left != nil, right != nil { return root }
+    else if left != nil { return left }
+    else { return right }
+  }
+}