diff --git a/Data Structures/isBST.js b/Data Structures/isBST.js
new file mode 100644
index 0000000..edb9742
--- /dev/null
+++ b/Data Structures/isBST.js	
@@ -0,0 +1,31 @@
+// https://www.freecodecamp.org/learn/coding-interview-prep/data-structures/check-if-tree-is-binary-search-tree
+
+var displayTree = (tree) => console.log(JSON.stringify(tree, null, 2));
+function Node(value) {
+  this.value = value;
+  this.left = null;
+  this.right = null;
+}
+function BinarySearchTree() {
+  this.root = null;
+}
+function isBinarySearchTree(tree) {
+  // Only change code below this line
+  if (tree.root === null) return true;
+  const stack = [tree.root];
+  while (stack.length) {
+    const node = stack.pop();
+    const leftChild = node.left;
+    if (leftChild !== null) {
+      stack.push(leftChild);
+      if (node.value <= leftChild.value) return false;
+    }
+    const rightChild = node.right;
+    if (rightChild !== null) {
+      stack.push(rightChild);
+      if (node.value > rightChild.value) return false;
+    }
+  }
+  return true;
+  // Only change code above this line
+}