diff --git a/W4_1_validated_bst.py b/W4_1_validated_bst.py
new file mode 100644
index 00000000..4e83b850
--- /dev/null
+++ b/W4_1_validated_bst.py
@@ -0,0 +1,29 @@
+class Solution:
+
+    def isValidBST(self, root):
+        self.result = True
+        self.prev = None
+        self.helper(root)
+        return self.result
+
+    def helper(self, root):
+
+        # Base
+        if root == None:
+            return 
+        
+        # Logic
+        if self.result:
+            self.helper(root.left)
+        
+        if self.prev is not None and self.prev.val >= root.val:
+            self.result = False
+
+        self.prev = root
+
+        if self.result:
+            self.helper(root.right)
+
+# Time complexity: O(N), worst case if the violation is in the right most last node, we visit/traverse all nodes
+# Space complexity: O(H), where h is the height of the tree (consider example of skewed tree)
+