diff --git a/construct_tree_pre_and_in.py b/construct_tree_pre_and_in.py
new file mode 100644
index 00000000..6a0c946e
--- /dev/null
+++ b/construct_tree_pre_and_in.py
@@ -0,0 +1,37 @@
+# Definition for a binary tree node.
+from typing import List
+
+
+class TreeNode:
+    def __init__(self, x):
+        self.val = x
+        self.left = None
+        self.right = None
+
+
+class Solution:
+    def buildTree(self, preorder: List[int], inorder: List[int]) -> TreeNode:
+
+        self.preorder_index = 0
+
+        # build a hashmap to store index and value for inorder list
+        inorder_index_map = {}
+        for index, value in enumerate(inorder):
+            inorder_index_map[value] = index
+
+        return self.helper(0, len(preorder) - 1, preorder, inorder_index_map)
+    
+    def helper(self, left, right, preorder, inorder_index_map):
+        if left > right:
+            return None
+
+        # select the preorder_index element as the root and increment it
+        root_value = preorder[self.preorder_index]
+        root = TreeNode(root_value)
+        self.preorder_index+=1
+        
+        # build left and right subtree
+        root.left = self.helper(left, inorder_index_map[root_value] - 1, preorder, inorder_index_map)
+        root.right = self.helper(inorder_index_map[root_value] + 1, right, preorder, inorder_index_map)
+
+        return root
\ No newline at end of file
diff --git a/validate_bst.py b/validate_bst.py
new file mode 100644
index 00000000..39a18176
--- /dev/null
+++ b/validate_bst.py
@@ -0,0 +1,25 @@
+# Definition for a binary tree node.
+from typing import Optional
+
+
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        return self.helper(root, float("-inf"), float("inf"))
+
+    def helper(self, root, low, high):
+        if not root:
+            return True
+
+        if root.val <= low or root.val >= high:
+            return False
+
+        return self.helper(root.right, root.val, high) and self.helper(
+            root.left, low, root.val
+        )