diff --git a/construct_binary_tree.py b/construct_binary_tree.py
new file mode 100644
index 00000000..3175e07e
--- /dev/null
+++ b/construct_binary_tree.py
@@ -0,0 +1,49 @@
+# Time Complexity : O(n)
+# Space complexity :O(n)
+# Did this code successfully run on Leetcode : Yes
+# Any problem you faced while coding this : Missed edge case and missed to have index variable at global level took some time to realize the mistake.
+
+# Your code here along with comments explaining your approach
+# Precompute a hash map of the inorder array values to their indices so that locating any root node and divide the remaining elements into left and right subtrees will be in O(1) time
+# Use a global tracking pointer (self.index) to get the next root node from the preorder array,move forward by one step every time a node is created.
+# Recursively build the left and right subtrees using start and end pointers.
+
+
+# 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]
+        """
+
+        self.index = 0
+        inorder_map = {}
+
+        for i in range(len(inorder)):
+            val = inorder[i]
+            inorder_map[val] = i
+
+        return self.helper(preorder, 0, len(inorder)-1, inorder_map)
+
+
+    def helper(self, preorder, start, end, inorder_map):
+        if start > end:
+            return None
+
+        root_val = preorder[self.index]
+        self.index += 1
+
+        root = TreeNode(root_val)
+
+        root_idx = inorder_map[root_val]
+        root.left = self.helper(preorder, start, root_idx - 1, inorder_map)
+        root.right = self.helper(preorder, root_idx + 1, end, inorder_map)
+
+        return root
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diff --git a/validate_BST.py b/validate_BST.py
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index 00000000..6013889e
--- /dev/null
+++ b/validate_BST.py
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+# Time Complexity : O(n^2)
+# Space complexity :O(1)
+# Did this code successfully run on Leetcode : Yes
+# Any problem you faced while coding this : Took lot of time to understand how recurrsion return works.
+
+# Your code here along with comments explaining your approach
+# 
+
+from collections import deque
+# 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 __init__(self):
+        self.prev = None
+
+    def isValidBST(self, root):
+        """
+        :type root: Optional[TreeNode]
+        :rtype: bool
+        """
+        return self.helper(root)
+
+    def helper(self, root):
+
+        if root is None:
+            return True
+        
+        if not self.helper(root.left):
+            return False
+
+        if self.prev is not None and self.prev.val >= root.val:
+            return False
+        
+        self.prev = root
+        return self.helper(root.right)
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