C#求组合数和组合的方法

    /// <summary>
    /// 组合数计算方法，两个参数可以互换，传入负数将会得到<c> 1 </c>；运算发生溢出将返回<c> 0 </c>。
    /// </summary>
    /// <param name="n">参数</param>
    /// <param name="N">参数</param>
    /// <returns>组合数。</returns>
    internal static ulong C(int n, int N)
    {
        System.Numerics.BigInteger ret = new System.Numerics.BigInteger(1);
        if (n != N && n > 1 && N > 1)
        {
            int m = N > n ? n : N;
            int M = N < n ? n : N;
            int c = M - m;
            c = c > m ? m : c;
            for (int i = M - c + 1; i <= M; i++)
                ret *= i;
            for (int i = 2; i <= c; i++)
                ret /= i;
            if (ret > ulong.MaxValue)
                ret = 0;
        }
        else if (n == 1 || N == 1)
        {
            ret = n > N ? n : N;
        }
        return (ulong)ret;
    }
    /// <summary>
    /// 组合计算方法，两个参数可以互换。
    /// </summary>
    /// <param name="n1">参数</param>
    /// <param name="n2">参数</param>
    /// <returns>组合。</returns>
    internal static int[][] CombList(int n1, int n2)
    {
        int[][] ret = new int[0][];
        if (n1 >= n2)
        {
            ret = Cv(n2, n1);
        }
        else if (n2 > n1)
        {
            ret = Cv(n1, n2);
        }
        return ret;
    }
    private static int[][] Cv(int n, int N)
    {
        int[][] ret = new int[0][];
        if (n > 0 && N > 0 && N >= n)
        {
            ulong count = C(n, N);//情况种数
            ret = new int[count][];
            ret[0] = new int[n];
            for (int i = 0; i < n; i++)
            {
                ret[0][i] = i;
            }
            for (ulong round = 1; round < count; round++)
            {
                int[] item = (int[])ret[round - 1].Clone();
                MaxValueGrow(item, N);
                ret[round] = item;
            }
        }
        return ret;
    }
    private static void MaxValueGrow(int[] arr, int max)
    {
        int count = arr.Length;
        int M = max - 1;
        for (int i = count - 1; i >= 0; i--, M--)
        {
            if (arr[i] < M)
            {
                arr[i]++;
                M = arr[i];
                for (; i < count; i++, M++)
                {
                    arr[i] = M;
                }
                break;
            }
        }
    }
来源：CSDN
作者：dscn15848078969
链接：https://blog.csdn.net/dscn15848078969/article/details/104415979
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