递归练习1，数字分组

/// <summary>
/// 将数字 sum 分为 k组，5分成3组，"0 0 5" ，"0 1 4" ，0 2 3 ...，2 1 2.
/// </summary>
/// <param name="sum"></param>
/// <param name="k"></param>
/// <returns>"0 0 5" ，"0 1 4" ，0 2 3 ...，2 1 2</returns>
public static List<string> C(int sum, int k) 
{
	List<string> result = new List<string>();
	C2(sum, k, "", result);
	return result;
}

private static void C2(int sum, int k, string str, List<string> result) 
{
	if (sum >= 0) 
	{
		if (k == 1) 
		{
			result.Add(str + " " + sum);
		} else 
		{
			for (int i = 0; i <= sum; i++) 
			{
				C2(sum - i, k - 1, str + " " + i, result);
			}
		}
	}
}