solves unique binary search trees

Author: anishLearnsToCode

README.md

@@ -1,7 +1,7 @@
 # LeetCode Algorithms
 
-![problems-solved](https://img.shields.io/badge/Problems%20Solved-460/2081-1f425f.svg)
-![problems-solved-java](https://img.shields.io/badge/Java-460/2081-1abc9c.svg)
+![problems-solved](https://img.shields.io/badge/Problems%20Solved-479/2081-1f425f.svg)
+![problems-solved-java](https://img.shields.io/badge/Java-479/2081-1abc9c.svg)
 ![problems-solved-python](https://img.shields.io/badge/Python-186/2081-1abc9c.svg)
 [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](CONTRIBUTING.md)
 [![cp](https://img.shields.io/badge/also%20see-Competitve%20Programming-1f72ff.svg)](https://github.com/anishLearnsToCode/competitive-programming)
@@ -86,7 +86,7 @@
 |  93  | [Restore IP Addresses](https://leetcode.com/problems/restore-ip-addresses)                                                                                |                                                               [![Java](assets/java.png)](src/RestoreIPAddresses.java)                                                                |                                                                                                                                   |
 |  94  | [Binary Tree Inorder Traversal](https://leetcode.com/problems/binary-tree-inorder-traversal)                                                              |                       [![Java](assets/java.png)](src/BinaryTreeInorderTraversal.java) [![Python](assets/python.png)](python/binary_tree_inorder_traversal.py)                        |                                                                                                                                   |
 |  95  | [Unique Binary Search Trees II](https://leetcode.com/problems/unique-binary-search-trees-ii)                                                              |                                                            [![Java](assets/java.png)](src/UniqueBinarySearchTreesII.java)                                                            |                                                                                                                                   |
-|  96  | [Unique Binary Search Trees](https://leetcode.com/problems/unique-binary-search-trees)                                                                    |                                                                                                                                                                                      |                                                                                                                                   |
+|  96  | [Unique Binary Search Trees](https://leetcode.com/problems/unique-binary-search-trees)                                                                    |                                                             [![Java](assets/java.png)](src/UniqueBinarySearchTrees.java)                                                             |                                                                                                                                   |
 |  97  | [Interleaving String](https://leetcode.com/problems/interleaving-string)                                                                                  |                                                                                                                                                                                      |                                                                                                                                   |
 |  98  | [Validate Binary Search Tree](https://leetcode.com/problems/validate-binary-search-tree)                                                                  |                                                                                                                                                                                      |                                                                                                                                   |
 |  99  | [Recover Binary Search Tree](https://leetcode.com/problems/recover-binary-search-tree)                                                                    |                                                                                                                                                                                      |                                                                                                                                   |

src/UniqueBinarySearchTrees.java

@@ -0,0 +1,17 @@
+// T: O(n)
+// S: O(1)
+
+public class UniqueBinarySearchTrees {
+    public int numTrees(int n) {
+        return catalanNumber(n);
+    }
+
+    private int catalanNumber(int n) {
+        long result = 1;
+        for (int i = 2 ; i <= n ; i++) {
+            result *= (4L * i - 2);
+            result /= i + 1;
+        }
+        return (int) result;
+    }
+}