diff --git a/chapter04/4.05 - Validate BST/validateBST.js b/chapter04/4.05 - Validate BST/validateBST.js
index c3e9a96..26bc875 100644
--- a/chapter04/4.05 - Validate BST/validateBST.js	
+++ b/chapter04/4.05 - Validate BST/validateBST.js	
@@ -4,23 +4,29 @@ var BinaryTree = function(value) {
   this.right = null;
 };
 
-var validateBST = function(bt) {
-  // traverse the tree depth first, while
-  // using a array with stack like behavior to check each node's validity
-  // * assume values do not repeat
-  var validateRecurse = function(currBt, stackArr) {
-    for (var i = 0; i < stackArr.length; i++) {
-      if (stackArr[i].side === 'left' && currBt.value > stackArr[i].node.value) {
-        return false;
-      } else if (stackArr[i].side === 'right' && currBt.value < stackArr[i].node.value) {
-        return false;
-      }
+var validateBST = function(tree) {
+
+  var validateRecurse = function(tree, min, max) {
+    if (tree === null) {
+      return true;
+    }
+
+    if (tree.value <= min || tree.value >= max) {
+      return false;
+    }
+
+    if (!validateRecurse(tree.left, min, tree.value)) {
+      return false;
+    }
+
+    if (!validateRecurse(tree.right, tree.value, max)) {
+      return false;
     }
-    var left = currBt.left === null ? true : validateRecurse(currBt.left, stackArr.concat([{ node:currBt, side:'left'}]));
-    var right = currBt.right === null ? true : validateRecurse(currBt.right, stackArr.concat([{ node:currBt, side:'right'}]));
-    return true && left && right;
+
+    return true;
   };
-  return validateRecurse(bt, []);
+  
+  return validateRecurse(tree, -Infinity, Infinity);
 };
 
 /* TESTS */