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. 1 3 3 2 1
\ / / / \ \
3 2 1 1 3 2
/ / \ \
2 1 2 3
Seen this question in a real interview before?class Solution {
public:
int numTrees(int n) {
int *res = new int[n+1];
res[0]=1;
res[1]=1;
for(int i=2;i<=n;i++){
res[i]=0;
for(int j=1;j<=i;j++)
res[i] += res[j-1] * res[i-j];
}
return res
;
}
};