LeetCode: Unique Binary Search Trees

Given n, how many structurally unique BST's (binary search trees) that store values 1...n?

For example,
Given n = 3, there are a total of 5 unique BST's.

   1         3     3      2      1
    \       /     /      / \      \
     3     2     1      1   3      2
    /     /       \                 \
   2     1         2                 3

   long long catalan(long long n)  
   {  
     if (n==1)return 1;  
     return ((2*(2*n-1)*catalan(n-1))/(n+1));  
   } 

 
   int numTrees(int n) {  
     if(n< 0)return 0;  
     return catalan(n);  
   }