Add solution #98

Author: JoshCrozier

README.md

@@ -44,6 +44,7 @@
 83|[Remove Duplicates from Sorted List](./0083-remove-duplicates-from-sorted-list.js)|Easy|
 88|[Merge Sorted Array](./0088-merge-sorted-array.js)|Easy|
 94|[Binary Tree Inorder Traversal](./0094-binary-tree-inorder-traversal.js)|Easy|
+98|[Validate Binary Search Tree](./0098-validate-binary-search-tree.js)|Medium|
 101|[Symmetric Tree](./0101-symmetric-tree.js)|Easy|
 102|[Binary Tree Level Order Traversal](./0102-binary-tree-level-order-traversal.js)|Medium|
 104|[Maximum Depth of Binary Tree](./0104-maximum-depth-of-binary-tree.js)|Easy|

solutions/0098-validate-binary-search-tree.js

@@ -0,0 +1,40 @@
+/**
+ * 98. Validate Binary Search Tree
+ * https://leetcode.com/problems/validate-binary-search-tree/
+ * Difficulty: Medium
+ *
+ * Given the root of a binary tree, determine if it is a valid binary search tree (BST).
+ *
+ * A valid BST is defined as follows:
+ * - The left subtree of a node contains only nodes with keys less than the node's key.
+ * - The right subtree of a node contains only nodes with keys greater than the node's key.
+ * - Both the left and right subtrees must also be binary search trees.
+ */
+
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val, left, right) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.left = (left===undefined ? null : left)
+ *     this.right = (right===undefined ? null : right)
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {boolean}
+ */
+var isValidBST = function(root) {
+  return traverse(root, null, null);
+};
+
+function traverse(root, min, max) {
+  if (!root) {
+    return true;
+  }
+
+  if ((min !== null && root.val <= min) || (max !== null && root.val >= max)) {
+    return false;
+  }
+
+  return traverse(root.left, min, root.val) && traverse(root.right, root.val, max);
+}