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

int calc(int n)

{

    if(n <= 1)

      return 1;

    int total = 0;

    for(int i = 1; i <= n; i++)

    {

       int left = calc(i-1);

       int right  = calc(n - i);

       total = total + left * right;

    }

    return total;

}

int numTrees(int n) {

  if(n <= 1)

      return n;

   return calc(n);

}