diff --git a/trres1.java b/trres1.java
new file mode 100644
index 00000000..e9d04e78
--- /dev/null
+++ b/trres1.java
@@ -0,0 +1,62 @@
+//problem1
+/**
+ * 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 Solution {
+    public boolean isValidBST(TreeNode root) {
+        if(root==null) return false;
+        isValidBST(root.left);
+        isValidBST(root.right);
+        if(root.val>root.left && root.val<root.right) return true;
+        return false;
+    }
+}
+//problem2
+/**
+ * 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 Solution {
+    int idx;
+    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);
+        }
+        this.idx = 0;
+        return helper(preorder, 0, preorder.length - 1, map);
+    }
+    public TreeNode helper(int[] preorder,int start,int end,HashMap<Integer,Integer> map){
+        if(start > end) return null;
+        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;
+    }
+}