feat: Add solution for LeetCode problem 98

Author: WhiteHyun

validate-binary-search-tree/WhiteHyun.swift

@@ -0,0 +1,45 @@
+//  
+//  98. Validate Binary Search Tree
+//  https://leetcode.com/problems/validate-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() { 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 {
+  func isValidBST(_ node: TreeNode?) -> Bool {
+    guard let node else { return true }
+
+    var left = node.left
+    var right = node.right
+
+    while left?.right != nil {
+      left = left?.right
+    }
+    while right?.left != nil {
+      right = right?.left
+    }
+
+    if left?.val ?? .min < node.val,
+       node.val < right?.val ?? .max {
+      return isValidBST(node.left) && isValidBST(node.right)
+    }
+
+    return false
+  }
+}