using System;  
using System.Collections.Generic;  
using System.Linq;  
using System.Text;  
using System.Threading.Tasks;  
  
namespace ConsoleApplication1  
{  
    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;  
        }  
    }  
}