diff --git a/build_with_preorder_inorder.py b/build_with_preorder_inorder.py
new file mode 100644
index 00000000..3264fd79
--- /dev/null
+++ b/build_with_preorder_inorder.py
@@ -0,0 +1,37 @@
+# Time Complexity: O(n)
+# Space Complexity: O(n) 
+# Did this code successfully run on Leetcode : Yes
+
+# Definition for a binary tree node.
+class TreeNode(object):
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+class Solution(object):
+    def buildTree(self, preorder, inorder):
+        """
+        :type preorder: List[int]
+        :type inorder: List[int]
+        :rtype: Optional[TreeNode]
+        """
+        hmap={}
+        self.rootindex=0
+        for i in range(len(inorder)):
+            hmap[inorder[i]]=i
+
+        def helper(in_st,in_end):
+            if (in_st>in_end): 
+                return None
+
+            rootval=preorder[self.rootindex]
+            in_root_idx=hmap[rootval]
+            root = TreeNode(rootval)
+            self.rootindex+=1
+            root.left = helper(in_st,in_root_idx-1)
+            root.right = helper(in_root_idx+1,in_end)
+
+            return root
+
+
+        return helper(0,len(preorder)-1)
\ No newline at end of file
diff --git a/validate_bst.py b/validate_bst.py
new file mode 100644
index 00000000..b97f0cb9
--- /dev/null
+++ b/validate_bst.py
@@ -0,0 +1,34 @@
+# Time Complexity: O(n)
+# Space Complexity: O(n) 
+# Did this code successfully run on Leetcode : Yes
+
+# Definition for a binary tree node.
+# class TreeNode(object):
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+
+class Solution(object):
+    def isValidBST(self, root):
+        """
+        :type root: Optional[TreeNode]
+        :rtype: bool
+        """
+        
+        self.flag=True
+        self.prev= None
+
+        def helper(root):
+            if root == None:
+                return
+            helper(root.left)
+            if self.prev!=None and root.val<=self.prev.val:
+                self.flag=False
+            self.prev=root
+            helper(root.right)       
+        
+        
+        helper(root)
+        return self.flag
+        
\ No newline at end of file