diff --git a/pre-in.py b/pre-in.py
new file mode 100644
index 00000000..853a6ed6
--- /dev/null
+++ b/pre-in.py
@@ -0,0 +1,73 @@
+# O(n^2) time bc .index takes O(n) and you do it N times
+# O(n^2) space $O(N^2)$While the recursion depth is $O(N)$, each call creates new list slices. Summing the memory of these slices across all levels of the recursion tree results in $O(N^2)$ total space used during the process.
+# 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 __init__(self, val=0,left=None,right=None):
+        self.val = val
+        self.left = left
+        self.right = right 
+
+    def buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
+        # both inorder and postorder are required to reconstruct the tree
+        # with just one and not the other you don't know which is root
+
+        # in order: left, root, right
+        # pre order: root, left, right
+
+        if not preorder:
+            return None
+        
+        rootVal = preorder[0] # 3
+        rootIdx = inorder.index(rootVal) # get the index of 3 in inorder
+
+        inLeft = inorder[:rootIdx] # inorder left subtree [8,9]
+        inRight = inorder[rootIdx+1:] # inorder right subtree [15,20,7]
+        preLeft = preorder[1:1+len(inLeft)] # [9,8]
+        preRight = preorder[1+len(inLeft):] # [20,15,7]
+
+        root = TreeNode(rootVal)
+        root.left = self.buildTree(preLeft, inLeft)
+        root.right= self.buildTree(preRight, inRight)
+
+        return root
+
+# How to optimize this to O(N)
+# To reach linear time and space, you need to stop copying the lists and stop searching for the index manually.
+# Use a Hash Map: Store the inorder values and their indices in a dictionary: {val: index}. 
+# This makes finding rootIdx an O(1) operation.Use Pointers: Instead of slicing lists (which creates copies), 
+# pass the start and end indices of the current subtree range.
+
+# use a hashmap with pointers
+class Solution:
+    def buildTree(self, preorder, inorder):
+        self.idx = 0
+        # Create an empty dictionary
+        inorder_map = {}
+
+        # Use a standard loop with enumerate to fill it: { val: i }
+        for i, val in enumerate(inorder):
+            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 
+        # move preorder index by 1 each time
+        rootVal = preorder[self.idx]
+        self.idx+=1
+        root = TreeNode(rootVal)
+        rootIdx = inorder_map[rootVal]
+        root.left = self.helper(preorder,start,rootIdx-1,inorder_map)
+        root.right = self.helper(preorder, rootIdx+1,end,inorder_map)
+
+        return root 
+
+
+
+
+
diff --git a/validate-bst.py b/validate-bst.py
new file mode 100644
index 00000000..c56c8515
--- /dev/null
+++ b/validate-bst.py
@@ -0,0 +1,58 @@
+# 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 __init__(self):
+        # pay attention to global vs parameter of recursion
+        # prev cannot go into the recursion stack
+        self.flag = True
+        self.prev = None
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        self.helper(root)
+        return self.flag
+    
+    # you cannot pass prev into the helper. Track what goes into the recursive stack only
+    # when recursion stack unfolds, the parameters of recursion will be overwritten
+    def helper(self,root):
+        if root == None:
+            return
+        # if I got a breach, exit - can save some of the nodes
+        if not self.flag:
+            return
+        print(root.val)
+        self.helper(root.left)
+        if self.prev != None and root.val <= self.prev.val:
+            self.flag = False
+            # even if you add a return here, all recursive calls are still completed because
+            # return returns to the same place from where we called it
+            # whatever goes into the recursion stack will come out
+        
+        self.prev = root
+        self.helper(root.right)
+
+
+# can also use range:
+
+class Solution:
+    def __init__(self):
+        # pay attention to global vs local scope. 
+        # prev cannot go into the recursion stack
+        self.flag = True
+        self.prev = None
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        self.helper(root,None,None)
+        return self.flag
+
+    def helper(self,root,min,max):
+        if root is None:
+            return
+        
+        self.helper(root.left,min,root.val)
+        if max != None and root.val >=max:
+            self.flag=False
+        if min != None and root.val <= min:
+            self.flag=False
+        self.helper(root.right,root.val,max)
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