Add validate-binary-search-tree solution

Author: Jeehay28

validate-binary-search-tree/Jeehay28.js

@@ -0,0 +1,29 @@
+// ✅ Time Complexity: O(N), where N is the number of nodes in the tree.
+// ✅ Space Complexity: O(N)
+
+/**
+ * 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) {
+  // Helper function to check BST validity
+  const dfs = (node, low, high) => {
+    // Base case: Empty subtree is valid
+    if (!node) return true;
+
+    if (!(low < node.val && node.val < high)) return false;
+
+    return dfs(node.left, low, node.val) && dfs(node.right, node.val, high);
+  };
+
+  return dfs(root, -Infinity, Infinity);
+};
+