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;
}
}