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) {
        return numTrees(1,n);
    }
private:
    int numTrees(int low,int high){
        int total = 0;
        if(low>high) return 0;
        if(low==high) return 1;
        for(int i=low;i<=high;++i){
            int left = numTrees(low,i-1);
            int right = numTrees(i+1,high);
            left = left==0 ? 1:left;
            right = right==0 ? 1:right;
            total += left*right;
        }
        return total;
    }
};