Given an Array of Interegers , NUmber of Unique Binary Searchj Trees Can be Made from this values are ?

you are building an N node binary search tree with the values 1..N. How many structurally different  binary search trees are there that store those values? Write a recursive function that, given the number of distinct values, computes the number of structurally unique binary search trees that store those values. For example, countTrees(4) should return 14, since there are 14  structurally unique binary search trees that store 1, 2, 3, and 4.