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

class Solution 
{
public:
    int numTrees(int n) 
	{
		vector<int> res;
		if(n == 0)
			return 0;
		if(n == 1)
			return 1;
		res.push_back(1);
		res.push_back(1);
		for(int i=2; i<=n; i++)
		{
			int temp = 0;
			for(int j=1; j<=i; j++)
				temp = temp + res[j-1]*res[i-j];
			res.push_back(temp);
		}
		return res[n];
    }
};

LeetCode--Unique Binary Search Trees