[Tree] add Solution to Validate Binary Search Tree

Tree/ValidateBinarySearchTree.swift

@@ -0,0 +1,38 @@
+/**
+ * Question Link: https://leetcode.com/problems/validate-binary-search-tree/
+ * Primary idea: Keep min to go right and keep max to go left
+ * 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 ValidateBinarySearchTree {
+    func isValidBST(root: TreeNode?) -> Bool {
+        return _helper(root, nil, nil)
+    }
+
+    private func _helper(node: TreeNode?, _ min: Int?, _ max: Int?) -> Bool {
+        guard let node = node else {
+            return true
+        }
+
+        if min != nil && node.val <= min {
+            return false
+        }
+        if max != nil && node.val >= max {
+            return false
+        }
+
+        return _helper(node.left, min, node.val) && _helper(node.right, node.val, max)
+    }
+}