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.

public class Solution {
    public int numTrees(int n) {
		// Start typing your Java solution below
		// DO NOT write main() function
		if (n == 0)
			return 1;
		if (n == 1)
			return 1;
		int total = 0;
		for (int i = 0; i < n; i++) {
			int left = numTrees(i);
			int right = numTrees(n - i - 1);
			total += left * right;
		}
		return total;
	}
}