C#求排列组合数

using System;
using System.Collections.Generic;

namespace Algorithms
{
    public class PermutationAndCombination
    {
        /// 

        /// 交换两个变量
        /// 
        /// 变量1
        /// 变量2
        public static void Swap(ref T a, ref T b)
        {
            T temp = a;
            a = b;
            b = temp;
        }

        /// 

        /// 递归算法求数组的组合(私有成员)
        /// 
        /// 返回的范型
        /// 所求数组
        /// 辅助变量
        /// 辅助变量
        /// 辅助数组
        /// 辅助变量M
        private static void GetCombination(ref List list, T[] t, int n, int m, int[] b, int M)
        {
            for (int i = n; i >= m; i--)
            {
                b[m - 1] = i - 1;
                if (m > 1)
                {
                    GetCombination(ref list, t, i - 1, m - 1, b, M);
                }
                else
                {
                    if (list == null)
                    {
                        list = new List();
                    }
                    T[] temp = new T[M];
                    for (int j = 0; j < b.Length; j++)
                    {
                        temp[j] = t[b[j]];
                    }
                    list.Add(temp);
                }
            }
        }

        /// 

        /// 递归算法求排列(私有成员)
        /// 
        /// 返回的列表
        /// 所求数组
        /// 起始标号
        /// 结束标号
        private static void GetPermutation(ref List list, T[] t, int startIndex, int endIndex)
        {
            if (startIndex == endIndex)
            {
                if (list == null)
                {
                    list = new List();
                }
                T[] temp = new T[t.Length];
                t.CopyTo(temp, 0);
                list.Add(temp);
            }
            else
            {
                for (int i = startIndex; i <= endIndex; i++)
                {
                    Swap(ref t[startIndex], ref t[i]);
                    GetPermutation(ref list, t, startIndex + 1, endIndex);
                    Swap(ref t[startIndex], ref t[i]);
                }
            }
        }
        
        /// 

        /// 求从起始标号到结束标号的排列，其余元素不变
        /// 
        /// 所求数组
        /// 起始标号
        /// 结束标号
        /// 从起始标号到结束标号排列的范型
        public static List GetPermutation(T[] t, int startIndex, int endIndex)
        {
            if (startIndex < 0 || endIndex > t.Length - 1)
            {
                return null;
            }
            List list = new List();
            GetPermutation(ref list, t, startIndex, endIndex);
            return list;
        }

        /// 

        /// 返回数组所有元素的全排列
        /// 
        /// 所求数组
        /// 全排列的范型
        public static List GetPermutation(T[] t)
        {
            return GetPermutation(t, 0, t.Length - 1);
        }

        /// 

        /// 求数组中n个元素的排列
        /// 
        /// 所求数组
        /// 元素个数
        /// 数组中n个元素的排列
        public static List GetPermutation(T[] t, int n)
        {
            if (n > t.Length)
            {
                return null;
            }
            List list = new List();
            List c = GetCombination(t, n);
            for (int i = 0; i < c.Count; i++)
            {
                List l = new List();
                GetPermutation(ref l, c[i], 0, n - 1);
                list.AddRange(l);
            }
            return list;
        }


        /// 

        /// 求数组中n个元素的组合
        /// 
        /// 所求数组
        /// 元素个数
        /// 数组中n个元素的组合的范型
        public static List GetCombination(T[] t, int n)
        {
            if (t.Length < n)
            {
                return null;
            }
            int[] temp = new int[n];
            List list = new List();
            GetCombination(ref list, t, t.Length, n, temp, n);
            return list;
        }
    }
} 

调用

int[] arr = new int[6];
for (int i = 0; i < arr.Length; i++)
{
    arr[i] = i + 1;
}
//求排列
List lst_Permutation = Algorithms.PermutationAndCombination.GetPermutation(arr, 3);
//求组合
List lst_Combination = Algorithms.PermutationAndCombination.GetCombination(arr, 3);