diff --git a/constructBTFromPreandInorder.cpp b/constructBTFromPreandInorder.cpp
new file mode 100644
index 00000000..21c3d976
--- /dev/null
+++ b/constructBTFromPreandInorder.cpp
@@ -0,0 +1,41 @@
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+
+ */
+class Solution {
+public:
+    unordered_map<int, int> um;
+    int preIndex;
+    TreeNode* helper(vector<int>& preorder, int left, int right) {
+        if (left > right) {
+            return nullptr;
+        }
+
+        TreeNode *root = new TreeNode(preorder[preIndex]);
+        preIndex++;
+
+        int index=um[root->val];
+
+        root->left = helper(preorder, left, index-1);
+        root->right = helper(preorder, index+1, right);
+
+        return root;
+    }
+    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
+        for (int i=0;i<inorder.size();i++) {
+            um[inorder[i]] = i;
+        }
+        this->preIndex = 0;
+        int left = 0;
+        int right = preorder.size()-1;
+        return helper(preorder, left, right);
+    }
+};
diff --git a/validateBST.cpp b/validateBST.cpp
new file mode 100644
index 00000000..0f4eee3f
--- /dev/null
+++ b/validateBST.cpp
@@ -0,0 +1,32 @@
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class Solution {
+public:
+    bool isValidBST(TreeNode* root) {
+        return helper(root, nullptr, nullptr);
+    }
+
+    bool helper(TreeNode* root, TreeNode* min, TreeNode* max) {
+        // termination condition
+        if (root == nullptr)
+            return true;
+
+        // logic
+        if (min != nullptr && root->val <= min->val) return false;
+        if (max != nullptr && root->val >= max->val) return false; 
+
+
+        //branching in the tree
+        return helper(root->left, min, root) && helper(root->right, root, max);
+
+    }
+};