diff --git a/ConstructBTFromPreAndIn.java b/ConstructBTFromPreAndIn.java
new file mode 100644
index 00000000..477fa087
--- /dev/null
+++ b/ConstructBTFromPreAndIn.java
@@ -0,0 +1,46 @@
+import java.util.*;
+
+// Recursion, O(n) time, O(h) space
+class Solution {
+    private int preIndex;
+
+    public TreeNode buildTree(int[] preorder, int[] inorder) {
+        preIndex = 0;
+        Map<Integer, Integer> map = new HashMap<>();
+
+        for (int i = 0; i < inorder.length; i++) {
+            map.put(inorder[i], i);
+        }
+
+        return helper(preorder, 0, inorder.length-1, map);
+    }
+
+    private TreeNode helper(int[] preorder, int start, int end, Map<Integer, Integer> map) {
+        if (start > end) return null;
+
+        TreeNode root = new TreeNode(preorder[preIndex]);
+        preIndex++;
+
+        int mid = map.get(root.val);
+        
+        root.left = helper(preorder, start, mid-1, map);
+        root.right = helper(preorder, mid+1, end, map);
+
+        return root;
+    }
+}
+
+
+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;
+    }
+}
+ 
\ No newline at end of file
diff --git a/ValidateBST.java b/ValidateBST.java
new file mode 100644
index 00000000..f0e08e7a
--- /dev/null
+++ b/ValidateBST.java
@@ -0,0 +1,32 @@
+// O(n) time, O(n) space
+class Solution {
+    public boolean isValidBST(TreeNode root) {
+        return helper(root, Long.MIN_VALUE, Long.MAX_VALUE);
+    }
+
+    private boolean helper(TreeNode root, long min, long max) {
+        if (root == null) return true;
+
+        if (root.val <= min || root.val >= max) {
+            return false;
+        }
+
+        boolean left = helper(root.left, min, root.val);
+        boolean right = helper(root.right, root.val, max);
+
+        return left && right;
+    }
+}
+
+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;
+    }
+}
\ No newline at end of file