Added UniqueBinarySearchTrees2 in java

Author: Anshuvats

Diff for: UniqueBinarySearchTrees2.java

@@ -0,0 +1,36 @@
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public List<TreeNode> generateTrees(int n) {
+        if(n == 0) return new LinkedList<TreeNode>();
+        return generateSubtrees(1, n);
+    }
+    
+    public List<TreeNode> generateSubtrees(int s, int e) {
+        List<TreeNode> res = new LinkedList<>();
+        if(s > e) {
+            res.add(null);
+            return res;
+        }
+        for(int i = s; i <= e; i++) {
+            List<TreeNode> left = generateSubtrees(s, i-1);
+            List<TreeNode> right = generateSubtrees(i+1, e);
+            for(TreeNode l: left) {
+                for(TreeNode r: right) {
+                    TreeNode n = new TreeNode(i);
+                    n.left = l;
+                    n.right = r;
+                    res.add(n);
+                }
+            }
+        }
+        return res;
+    }
+}