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

 

 

class Solution {
public:
    int numTrees(int n) {
        if(n <= 1) return 1;
        int ret = 0;
        for(int i = 0; i < n; ++i) {
            ret += numTrees(i) * numTrees(n-i-1);
        }
        return ret;
    }
};