unique-binary-search-trees Java code

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){
return -1;
}
int[] dp=new int[n+1];
dp[0]=1;
dp[1]=1;
for(int i=2;i<=n;i++){
for(int j=0;j<i;j++){
dp[i]+=dp[j]*dp[i-j-1];
}
}
return dp
;
}
}