diff --git a/constructTree.py b/constructTree.py
new file mode 100644
index 00000000..3349ab83
--- /dev/null
+++ b/constructTree.py
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+# Time Complexity : O(n)
+# Space Complexity : O(n)
+# Did this code successfully run on Leetcode : Yes
+# Any problem you faced while coding this : No
+# Approach:
+# Use preorder to pick root nodes sequentially (root -> left -> right).
+# Use a hashmap to store (value : index) for inorder traversal for O(1) lookup.
+# Maintain a pointer (pre_idx) to track current root in preorder.
+# For each root, find its index in inorder to divide into left and right subtrees.
+# Recursively build left subtree first, then right subtree using inorder boundaries.
+
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+
+class Solution:
+    def buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
+        inorder_index = {val: i for i, val in enumerate(inorder)}
+        pre_idx = 0
+
+        def helper(left, right):
+            nonlocal pre_idx
+
+            if left > right:
+                return None
+
+            root_val = preorder[pre_idx]
+            pre_idx += 1
+
+            root = TreeNode(root_val)
+            mid = inorder_index[root_val]
+
+            root.left = helper(left, mid - 1)
+            root.right = helper(mid + 1, right)
+
+            return root
+
+        return helper(0, len(inorder) - 1)
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diff --git a/validateBST.py b/validateBST.py
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index 00000000..403c7493
--- /dev/null
+++ b/validateBST.py
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+# Time Complexity : O(n)
+# Space Complexity : O(n)
+# Did this code successfully run on Leetcode : Yes
+# Any problem you faced while coding this : No
+# Approach: 
+# Preorder gives the root first. Use a hashmap to store (value : index) for inorder traversal.
+# Maintain a pointer for preorder index. 
+# For each root, find its position in inorder to split into left and right subtrees.
+# Recursively build left subtree first, then right subtree.
+
+class Solution:
+    def buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
+        inorder_index = {val: i for i, val in enumerate(inorder)}
+        pre_idx = 0
+
+        def helper(left, right):
+            nonlocal pre_idx
+
+            if left > right:
+                return None
+
+            root_val = preorder[pre_idx]
+            pre_idx += 1
+
+            root = TreeNode(root_val)
+            mid = inorder_index[root_val]
+
+            root.left = helper(left, mid - 1)
+            root.right = helper(mid + 1, right)
+
+            return root
+
+        return helper(0, len(inorder) - 1)
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