O(n) -> O(log n) for validate BST

Author: jotshin

README.md

@@ -163,7 +163,7 @@
 [Balanced Binary Tree](https://leetcode.com/problems/balanced-binary-tree/)| [Swift](./Tree/BalancedBinaryTree.swift)| Easy|  O(n)| O(n)|
 [Sum of Left Leaves](https://leetcode.com/problems/sum-of-left-leaves/)| [Swift](./Tree/SumLeftLeaves.swift)| Easy|  O(n)| O(1)|
 [Flatten Binary Tree to Linked List](https://leetcode.com/problems/flatten-binary-tree-to-linked-list/)| [Swift](./Tree/FlattenBinaryTreeLinkedList.swift)| Medium| O(n)| O(1)|
-[Validate Binary Search Tree](https://leetcode.com/problems/validate-binary-search-tree/)| [Swift](./Tree/ValidateBinarySearchTree.swift)| Medium| O(n)| O(n)|
+[Validate Binary Search Tree](https://leetcode.com/problems/validate-binary-search-tree/)| [Swift](./Tree/ValidateBinarySearchTree.swift)| Medium| O(n)| O(log n)|
 [Binary Tree Level Order Traversal](https://leetcode.com/problems/binary-tree-level-order-traversal/)| [Swift](./Tree/BinaryTreeLevelOrderTraversal.swift)| Easy| O(n)| O(n)|
 [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)|

Tree/ValidateBinarySearchTree.swift

@@ -1,7 +1,7 @@
 /**
  * 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(n)
+ * Time Complexity: O(n), Space Complexity: O(log n)
  * 
  * Definition for a binary tree node.
  * public class TreeNode {
@@ -35,4 +35,4 @@ class ValidateBinarySearchTree {
 
         return _helper(node.left, min, node.val) && _helper(node.right, node.val, max)
     }
-}
+}