diff --git a/ConstructBinaryTree.java b/ConstructBinaryTree.java
new file mode 100644
index 00000000..b4da2e51
--- /dev/null
+++ b/ConstructBinaryTree.java
@@ -0,0 +1,50 @@
+//Time Complexity : O(n)
+//Space Complexity : O(n)
+
+ /* Definition for a binary tree node.
+  * public class TreeNode {
+  *     int val;
+  *     TreeNode left;
+  *     TreeNode right;
+  *     TreeNode() {}
+  *     TreeNode(int val) { this.val = val; }
+  *     TreeNode(int val, TreeNode left, TreeNode right) {
+  *         this.val = val;
+  *         this.left = left;
+  *         this.right = right;
+  *     }
+  * }
+  */
+
+
+
+ import java.util.HashMap;
+
+ public class ConstructBinaryTree {
+     int index;
+    public TreeNode buildTree(int[] preorder, int[] inorder) {
+        HashMap<Integer, Integer> map = new HashMap<>();
+        for(int i = 0; i < inorder.length; i++){
+            map.put(inorder[i], i);
+        }
+        index = 0;
+        return helper(preorder, 0, preorder.length - 1, map);
+    }
+
+    private TreeNode helper (int[] preorder, int start, int end, HashMap<Integer, Integer> map) {
+
+        if(start > end) return null;
+
+        int rootVal = preorder[index];
+        index++;
+
+        int rootIndex = map.get(rootVal);
+
+        TreeNode root = new TreeNode(rootVal);
+
+        root.left = helper(preorder, start, rootIndex - 1, map);
+        root.right = helper(preorder, rootIndex + 1, end, map);
+        return root;
+    }
+
+}
diff --git a/ValidateBST.java b/ValidateBST.java
new file mode 100644
index 00000000..c7fd6725
--- /dev/null
+++ b/ValidateBST.java
@@ -0,0 +1,31 @@
+//Time Complexity: O(n)
+//Space Complexity: O(h)
+// Definition for a binary tree node.
+class TreeNode {
+      int val;
+      TreeNode left;
+      TreeNode right;
+      TreeNode() {}
+      TreeNode(int val) { this.val = val; }
+      TreeNode(int val, TreeNode left, TreeNode right) {
+          this.val = val;
+          this.left = left;
+          this.right = right;
+      }
+ }
+
+
+public class ValidateBST {
+
+    public boolean isValidBST(TreeNode root) {
+        return compareNode(root, null, null);
+    }
+
+    private boolean compareNode (TreeNode node, TreeNode lower, TreeNode upper) {
+        if(node == null) return true;
+        if(lower!= null && node.val<= lower.val) return false;
+        if(upper != null && node.val >= upper.val) return false;
+        return compareNode(node.left,lower,node) && compareNode(node.right,node,upper);
+
+    }
+}