diff --git a/Problem43.java b/Problem43.java
new file mode 100644
index 00000000..30861354
--- /dev/null
+++ b/Problem43.java
@@ -0,0 +1,25 @@
+// Time Complexity: O(N)
+// Space Complexity: O(h) - height/depth of the tree
+
+class Solution {
+    private boolean flag;
+    private TreeNode prev;
+    public boolean isValidBST(TreeNode root) {
+        this.flag = true;
+        helper(root);
+        return flag;
+    }
+
+    private void helper(TreeNode root) {
+        //base
+        if(root == null) return;
+
+        //logic
+        helper(root.left);
+        if (this.prev != null && this.prev.val >= root.val) {
+            this.flag = false;
+        }
+        prev = root;
+        helper(root.right);
+    }
+}
\ No newline at end of file
diff --git a/Problem44.java b/Problem44.java
new file mode 100644
index 00000000..6fec812b
--- /dev/null
+++ b/Problem44.java
@@ -0,0 +1,31 @@
+// Time Complexity: O(N)
+// Space Complexity: O(N)
+class Solution {
+    private int idx;
+    public TreeNode buildTree(int[] preorder, int[] inorder) {
+        Map<Integer, Integer> map = new HashMap<>();
+        for (int i = 0; i < inorder.length; i++) {
+            map.put(inorder[i],i);
+        }
+        this.idx = 0;
+        return helper(preorder, 0, preorder.length-1, map);
+    }
+
+    private TreeNode helper(int[] preorder, int start, int end, Map<Integer, Integer> map) {
+        //base
+        if (start > end) return null;
+
+        //logic
+        int rootVal = preorder[idx];
+        idx++;
+
+        int rootIdx = map.get(rootVal);
+
+        TreeNode root = new TreeNode(rootVal);
+
+        root.left = helper(preorder, start, rootIdx-1, map);
+        root.right = helper(preorder, rootIdx+1, end, map);
+
+        return root;
+    }
+}
\ No newline at end of file