【LeetCode OJ】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

public class Solution {
    public int numTrees(int n) {
		if(n == 0 || n == 1)return 1;
		int[] f = new int[n+1];
		f[0] = f[1] = 1;
		for(int i = 2; i < n + 1; ++i){
			for(int j = 1; j <= i; ++j){
				f[i] += f[i - j] * f[j - 1];
			}
		}
		return f[n];        
    }
}