using System;
using System.Collections.Generic;

namespace Algorithms
{
    
public class PermutationAndCombination<T>
    {
        
/// <summary>
        
/// 交换两个变量
        
/// </summary>
        
/// <param name="a">变量1</param>
        
/// <param name="b">变量2</param>
        public static void Swap(ref T a, ref T b)
        {
            T temp 
= a;
            a 
= b;
            b 
= temp;
        }

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

        
/// <summary>
        
/// 递归算法求排列(私有成员)
        
/// </summary>
        
/// <param name="list">返回的列表</param>
        
/// <param name="t">所求数组</param>
        
/// <param name="startIndex">起始标号</param>
        
/// <param name="endIndex">结束标号</param>
        private static void GetPermutation(ref List<T[]> list, T[] t, int startIndex, int endIndex)
        {
            
if (startIndex == endIndex)
            {
                
if (list == null)
                {
                    list 
= new List<T[]>();
                }
                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]);
                }
            }
        }
        
        
/// <summary>
        
/// 求从起始标号到结束标号的排列,其余元素不变
        
/// </summary>
        
/// <param name="t">所求数组</param>
        
/// <param name="startIndex">起始标号</param>
        
/// <param name="endIndex">结束标号</param>
        
/// <returns>从起始标号到结束标号排列的范型</returns>
        public static List<T[]> GetPermutation(T[] t, int startIndex, int endIndex)
        {
            
if (startIndex < 0 || endIndex > t.Length - 1)
            {
                
return null;
            }
            List
<T[]> list = new List<T[]>();
            GetPermutation(
ref list, t, startIndex, endIndex);
            
return list;
        }

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

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


        
/// <summary>
        
/// 求数组中n个元素的组合
        
/// </summary>
        
/// <param name="t">所求数组</param>
        
/// <param name="n">元素个数</param>
        
/// <returns>数组中n个元素的组合的范型</returns>
        public static List<T[]> GetCombination(T[] t, int n)
        {
            
if (t.Length < n)
            {
                
return null;
            }
            
int[] temp = new int[n];
            List
<T[]> list = new List<T[]>();
            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<int[]> lst_Permutation = Algorithms.PermutationAndCombination<int>.GetPermutation(arr, 3);
//求组合
List<int[]> lst_Combination = Algorithms.PermutationAndCombination<int>.GetCombination(arr, 3);