Add leetcode #98 solution (#65)

Co-authored-by: Chi Hi Ong <[email protected]>
Co-authored-by: Gourav Rusiya <[email protected]>

Author: 3 people

JavaScript/98.Validate-Binary-Search-Tree.js

@@ -0,0 +1,52 @@
+/**
+ * 
+ * Given a binary tree, determine if it is a valid binary search tree (BST).
+ * 
+ * Assume a 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.
+ *
+ * Example 1:
+
+    2
+   / \
+  1   3
+
+Input: [2,1,3]
+Output: true
+Example 2:
+
+    5
+   / \
+  1   4
+     / \
+    3   6
+
+Input: [5,1,4,null,null,3,6]
+Output: false
+Explanation: The root node's value is 5 but its right child's value is 4.
+ */
+
+var isValidBST = function (
+  root,
+  lower = Number.MIN_SAFE_INTEGER,
+  upper = Number.MAX_SAFE_INTEGER
+) {
+  if (!root) return true;
+
+  let value = root.val;
+  if (value <= lower || value >= upper) {
+    return false;
+  }
+
+  if (!isValidBST(root.right, value, upper)) {
+    return false;
+  }
+
+  if (!isValidBST(root.left, lower, value)) {
+    return false;
+  }
+  return true;
+};

README.md

@@ -189,7 +189,7 @@ Check out ---> [Sample PR](https://github.com/codedecks-in/LeetCode-Solutions/pu
 | 124  | [Binary Tree Maximum Path Sum](https://leetcode.com/problems/binary-tree-maximum-path-sum/)                     | [C++](./C++/Binary-Tree-Maximum-Path-Sum.cpp)                                 | _O(n)_    | _O(n)_    | Hard       | Tree                                           |      |
 | 1028 | [Recover a Tree From Preorder Traversal](https://leetcode.com/problems/recover-a-tree-from-preorder-traversal/) | [C++](./C++/Recover-a-Tree-From-Preorder-Traversal.cpp)                       | _O(n)_    | _O(n)_    | Hard       | Tree                                           |      |
 | 968 | [Binary Tree Cameras](https://leetcode.com/problems/binary-tree-cameras/)     | [C++](./C++/Binary-Tree-Cameras.cpp)          | _O(n)_    | _O(logn)_    | Hard     | Binary Tree, Dynamic Programming  
-  
+| 98 | [Validate Binary Search Tree](https://leetcode.com/problems/validate-binary-search-tree/)     | [Javascript](./JavaScript/98.Validate-Binary-Search-Tree.js)          | _O(log(n))_    | _O(log(n))_    | Medium     | Binary Tree  
 
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