LeetCode之numTrees

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