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==0 || n==1) return 1;
        
        int sum=0;
        for(int i=1;i<=n;i++)
            sum=sum+numTrees(i-1)*numTrees(n-i);
        return sum;    
    }
};