diff --git a/BuildTree.java b/BuildTree.java
new file mode 100644
index 00000000..10047e6a
--- /dev/null
+++ b/BuildTree.java
@@ -0,0 +1,33 @@
+public class BuildTree {
+    private Map<Integer, Integer> inorderMap = new HashMap<>();
+    private int preorderIndex = 0;
+    // Time complexity: O(n)
+    // Space complexity: O(h)
+    // Ran on Leetcode: yes
+    public TreeNode buildTree(int[] preorder, int[] inorder) {
+
+        for (int i = 0; i < inorder.length; i++) {
+            inorderMap.put(inorder[i], i);
+        }
+
+        return helper(preorder, 0, inorder.length - 1);
+    }
+
+    private TreeNode helper(int[] preorder, int left, int right) {
+        if (left > right) {
+            return null;
+        }
+
+        int rootVal = preorder[preorderIndex];
+        preorderIndex++;
+        TreeNode root = new TreeNode(rootVal);
+
+        int inorderIndex = inorderMap.get(rootVal);
+
+        root.left = helper(preorder, left, inorderIndex - 1);
+
+        root.right = helper(preorder, inorderIndex + 1, right);
+
+        return root;
+    }
+}
\ No newline at end of file
diff --git a/ValidBST.java b/ValidBST.java
new file mode 100644
index 00000000..30fe1297
--- /dev/null
+++ b/ValidBST.java
@@ -0,0 +1,16 @@
+class ValidBST {
+    // Time: O(n)
+    // Space: O(h)
+    // Ran on Leetcode: yes
+    public boolean isValidBST(TreeNode root) {
+        return helper(root, null, null);
+    }
+
+    private boolean helper(TreeNode root, Integer min, Integer max) {
+        if(root == null ) return true;
+        if ((min != null && min >= root.val) ||  (max != null && max <= root.val)) {
+            return false;
+        }
+        return helper(root.left, min, root.val) && helper(root.right, root.val, max);
+    }
+}
\ No newline at end of file