diff --git a/ConstructBTFromInorderAndPreorder.swift b/ConstructBTFromInorderAndPreorder.swift
new file mode 100644
index 00000000..3c9d0002
--- /dev/null
+++ b/ConstructBTFromInorderAndPreorder.swift
@@ -0,0 +1,84 @@
+//
+//  ConstructBinaryTreeFromPreorder.swift
+//  DSA-Practice
+//
+//  Created by Paridhi Malviya on 1/19/26.
+//
+
+class BinaryTreeFromInorderPreorder {
+    
+    init() {
+        let preOrder = [3,9,20,15,7]
+        let inorder = [9,3,15,20,7]
+//        let _ = constructTreeUsingSubarrays(preOrder, inorder)
+        var inorderMap = Dictionary<Int, Int>()
+        for (index, element) in inorder.enumerated() {
+            inorderMap[element] = index
+        }
+        // return constructTreeUsingSubarrays(preorder, inorder)
+        let _ = helper(preOrder, inMap: inorderMap, inStart: 0, inEnd: inorder.count - 1)
+
+    }
+    
+    /*
+     time compelxity - n recursive calls. O(n)
+     2. in each recursive call - we are doing n calls for searching the inorder root - O(n)
+     3. creating deep copies - O(n) Array(...)
+     2nd and 3rd needs O(n) time in each recursive call
+     So total time compelxity - O(n^2)
+     
+     Space complexity - O(n^2)
+     n recursive calls. In each, creating four parts of the same array O(n)
+     */
+    func constructTreeUsingSubarrays(_ preorder: [Int], _ inorder: [Int]) -> TreeNode? {
+        
+        if (preorder.count == 0) {
+            return nil
+        }
+        let rootVal = preorder[0]
+        let rootNode = TreeNode(val: rootVal, left: nil, right: nil)
+
+        var inorderRoot = -1
+        //find out the value in inorder array
+        for i in 0..<inorder.count {
+            if (inorder[i] == rootVal) {
+                inorderRoot = i
+                break
+            }
+        }
+
+        let inLeft = inorder[0..<inorderRoot] //span of inorder of left subtree
+        let inRight = inorder[(inorderRoot+1)..<(inorder.count)]
+        let preLeft = preorder[1..<1+inLeft.count]
+        let preRight = preorder[1+inLeft.count..<preorder.count]
+
+
+        rootNode.left = constructTreeUsingSubarrays(Array(preLeft), Array(inLeft))
+        rootNode.right = constructTreeUsingSubarrays(Array(preRight), Array(inRight))
+        return rootNode
+
+    }
+    
+    /*
+     Time complexity - O(n)
+     space complexity - O(n)
+     */
+    var preOrderIdx: Int  = 0
+    
+    //using pointers and map
+    func helper(_ preorder: [Int], inMap: Dictionary<Int,Int>, inStart: Int, inEnd: Int) -> TreeNode? {
+        if (inStart > inEnd) {
+            return nil
+        }
+        //logic
+        let rootVal =  preorder[preOrderIdx]
+        let root = TreeNode(val: rootVal, left: nil, right: nil)
+        preOrderIdx = preOrderIdx + 1
+
+        let inRIdx = inMap[rootVal]!
+
+        root.left = helper(preorder, inMap: inMap, inStart: inStart, inEnd: inRIdx - 1)
+        root.right = helper(preorder, inMap: inMap, inStart: inRIdx + 1, inEnd: inEnd)
+        return root
+    }
+}
diff --git a/ValidateBinarySearchTree.swift b/ValidateBinarySearchTree.swift
new file mode 100644
index 00000000..a464db66
--- /dev/null
+++ b/ValidateBinarySearchTree.swift
@@ -0,0 +1,159 @@
+//
+//  Validate binary search tree.swift
+//  DSA-Practice
+//
+//  Created by Paridhi Malviya on 1/15/26.
+//
+
+/*
+ inorder traversal is sorted if it is BST.
+ inorder - left -> root -> right, right -> root -> left
+ preorder = root -> left -> right , root -> right -> left
+ postorder = left - right - root, right - left - root
+
+ to check if it is valid bst or not -> Do inorder, if sorted, it's BST
+ */
+
+class TreeNode {
+    var left: TreeNode?
+    var right: TreeNode?
+    var val: Int
+    
+    init(val: Int, left: TreeNode?, right: TreeNode?) {
+        self.val = val
+        self.left = left
+        self.right = right
+    }
+}
+
+class BinarySearchTree {
+    
+    var prev: TreeNode?
+    var flag: Bool
+    
+    init() {
+        flag = true
+    }
+    
+    func inorderTraversal(_ root: TreeNode?) -> [Int] {
+        //base
+        if (root == nil) {
+            return []
+        }
+        let leftTraversal: [Int] = inorderTraversal(root?.left)
+        let rightTraversal = inorderTraversal(root?.right)
+        return leftTraversal + [root!.val] + rightTraversal
+    }
+    
+    func isValidBST(root: TreeNode?) -> Bool {
+        self.flag = true
+        inorderToCheckValidBST(root)
+        return flag
+    }
+    
+    func inorderToCheckValidBST(_ root: TreeNode?) {
+        if (root == nil) {
+            return
+        }
+        if (!flag) {
+            return
+        }
+        inorderToCheckValidBST(root?.left)
+        if (prev != nil && root!.val <= prev!.val) {
+            //breach
+            flag = false
+        }
+        prev = root
+            inorderToCheckValidBST(root?.right)
+    }
+    
+    //without flag bool
+    func isValidBSTWithoutFLag(root: TreeNode?) -> Bool {
+        return inorderToCheckValidBSTBooleanBased(root)
+    }
+    
+    //boolean based recursion
+    func inorderToCheckValidBSTBooleanBased(_ root: TreeNode?) -> Bool {
+        if (root == nil) {
+            return true
+        }
+        let isLeftValid: Bool = inorderToCheckValidBSTBooleanBased(root?.left)
+        if (!isLeftValid) {
+            return false
+        }
+        
+        if (prev != nil && root!.val <= prev!.val) {
+            //breach
+            return false
+        }
+        prev = root
+        return inorderToCheckValidBSTBooleanBased(root?.right)
+    }
+}
+
+extension BinarySearchTree {
+    //Without previous variable
+    
+    func inOrderWithoutPreviousVariable(_ root: TreeNode?) -> Bool {
+        if (root == nil) {
+            return true
+        }
+        
+        let isLeftValid = inOrderWithoutPreviousVariable(root?.left)
+        let isRightValid = inOrderWithoutPreviousVariable(root?.right)
+        
+        if (root!.left != nil && root!.val < root!.left!.val) {
+            return false
+        }
+        
+        if (root!.right != nil && root!.val > root!.right!.val) {
+            return false
+        }
+        
+        return isLeftValid && isRightValid
+    }
+}
+/*
+ at every node, we can have a potential range. Each element should lie in that range. If there is a breach, then it's not a valid BST.
+ start recursion from the root.
+ IF  we keep max and min in global then,
+ //if going left, then max = root.val, minimum = earlier minimum only
+ 
+ If I go right , then minimum will be root.val. maximum = earlier maximum
+ Every node should have it's own minimum and maximum.
+ 
+ time complexity -> each node will be visited once so O(n)
+ */
+class BinarySearchTree2 {
+    
+    var flag = true
+    var node: TreeNode?
+    init() {
+        
+        let isValid = validateBinarySearchUsingRange(root: node, min: Int.min, max: Int.max)
+        print("isValid \(isValid)")
+    }
+    
+    func validateBinarySearchUsingRange(root: TreeNode?, min: Int, max: Int) -> Bool {
+        
+        if (root == nil) {
+            return true
+        }
+        
+        //checking that the node is in the range at preorder level. We can check the same at postorder and inorder level.
+        /*minimum and maximum are in the recursion stack. Whenever root is coming out, it's comin gout with it's minimum and maximum. It will remain same at the preorder, postorder and inorder*/
+        if (root!.val >= max && root!.val <= min) {
+            return false
+        }
+        
+        let isLeftValid = validateBinarySearchUsingRange(root: root?.left, min: min, max: root!.val)
+        
+        //stopping the right side recursive calls id the flag is false.
+        if (!isLeftValid) {
+            return false
+        }
+        let isRightValid = validateBinarySearchUsingRange(root: root?.right, min: root!.val, max: max)
+        
+        return isLeftValid && isRightValid
+    }
+}