【LeetCode】Unique Binary Search Trees

int numTrees(int n)
{
	int i, k;
	vector<int> f(n + 1, 0);
	f[0] = 1; f[1] = 1;
	for (i = 2; i <= n; i++)
	{
		for (k = 1; k <=i; k++)
		{
			f[i] += f[k - 1] * f[i - k];
		}
	}
	return f[n];
}