diff --git a/Problem1.java b/Problem1.java
new file mode 100644
index 00000000..2851fddb
--- /dev/null
+++ b/Problem1.java
@@ -0,0 +1,38 @@
+/**
+ * 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;
+ *     }
+ * }
+ */
+class Problem1 {
+    boolean flag;
+    TreeNode prev;
+    
+    public boolean isValidBST(TreeNode root) {
+        this.flag = true;
+        helper(root);
+        return flag;
+    }
+
+    private void helper(TreeNode root){
+        if(root == null) return;
+
+        helper(root.left);
+
+        if(prev != null && prev.val >= root.val){
+            flag = false;
+        }
+
+        prev = root;
+        helper(root.right);
+    }
+}
diff --git a/Problem2.java b/Problem2.java
new file mode 100644
index 00000000..eefb7839
--- /dev/null
+++ b/Problem2.java
@@ -0,0 +1,43 @@
+/**
+ * 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;
+ *     }
+ * }
+ */
+class Problem2 {
+    public TreeNode buildTree(int[] preorder, int[] inorder) {
+
+        // base case
+        if(preorder.length == 0) return null;
+        
+        int rootVal = preorder[0];
+
+        int rootIdx = -1;
+        for(int i = 0; i < inorder.length; i++){
+            if(inorder[i] == rootVal){
+                rootIdx = i;
+                break;
+            }
+        }
+
+        int[] inLeft = Arrays.copyOfRange(inorder, 0, rootIdx);
+        int[] inRight = Arrays.copyOfRange(inorder, rootIdx + 1, inorder.length);
+        int[] preLeft = Arrays.copyOfRange(preorder, 1, inLeft.length + 1);
+        int[] preRight = Arrays.copyOfRange(preorder, inLeft.length + 1, preorder.length);
+
+        TreeNode root = new TreeNode(rootVal);
+        root.left = buildTree(preLeft, inLeft);
+        root.right = buildTree(preRight, inRight);
+
+        return root;
+    }
+}