Fix Unique Binary Search Tree Time Complexity

Author: soapyigu

README.md

@@ -152,7 +152,7 @@
 [Path Sum](https://leetcode.com/problems/path-sum/)| [Swift](./Tree/PathSum.swift)| Easy| O(n)| O(n)|
 [Path Sum II](https://leetcode.com/problems/path-sum-ii/)| [Swift](./Tree/PathSumII.swift)| Medium| O(n)| O(n)|
 [Path Sum III](https://leetcode.com/problems/path-sum-iiI/)| [Swift](./Tree/PathSumIII.swift)| Easy| O(n^2)| O(1)|
-[Unique Binary Search Trees](https://leetcode.com/problems/unique-binary-search-trees/)| [Swift](./Tree/UniqueBinarySearchTrees.swift)| Medium| O(n)| O(n)|
+[Unique Binary Search Trees](https://leetcode.com/problems/unique-binary-search-trees/)| [Swift](./Tree/UniqueBinarySearchTrees.swift)| Medium| O(n^2)| O(n)|
 [Recover Binary Search Tree](https://leetcode.com/problems/recover-binary-search-tree/)| [Swift](./Tree/RecoverBinarySearchTree.swift)| Hard| O(n)| O(1)|
 
 ## Dynamic programming