leetcode 96 Unique Binary Search Trees ----- java

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

 给一个正整数n，然后将这n个数进行二叉排序树的排列，求有多少种组合。

public class Solution {
    public int numTrees(int n){ 
        if( n < 3)
            return n;
        int[] dp = new int[n+1];
        dp[0] = 1;
        dp[1] = 1;
        dp[2] = 2;
        for( int i = 3;i<n+1;i++){
            for( int j = 0;j<i;j++){
                dp[i] += dp[j]*dp[i-j-1];
            }
        }
        return dp[n];    
    }
}