using System;
using System.Collections.Generic;

namespace Algorithms
{
    public class PermutationAndCombination<T>
    {
        /// <summary>
        /// 交換兩個變數
        /// </summary>
        /// <param name="a">變數1</param>
        /// <param name="b">變數2</param>
        public static void Swap(ref T a, ref T b)
        {
            T temp = a;
            a = b;
            b = temp;
        }

        /// <summary>
        /// 遞迴演算法求陣列的組合(私有成員)
        /// </summary>
        /// <param name="list">返回的範型</param>
        /// <param name="t">所求陣列</param>
        /// <param name="n">輔助變數</param>
        /// <param name="m">輔助變數</param>
        /// <param name="b">輔助陣列</param>
        /// <param name="M">輔助變數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>
        /// 遞迴演算法求排列(私有成員)
        /// </summary>
        /// <param name="list">返回的列表</param>
        /// <param name="t">所求陣列</param>
        /// <param name="startIndex">起始標號</param>
        /// <param name="endIndex">結束標號</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>
        /// 求從起始標號到結束標號的排列，其餘元素不變
        /// </summary>
        /// <param name="t">所求陣列</param>
        /// <param name="startIndex">起始標號</param>
        /// <param name="endIndex">結束標號</param>
        /// <returns>從起始標號到結束標號排列的範型</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>
        /// 返回陣列所有元素的全排列
        /// </summary>
        /// <param name="t">所求陣列</param>
        /// <returns>全排列的範型</returns>
        public static List<T[]> GetPermutation(T[] t)
        {
            return GetPermutation(t, 0, t.Length - 1);
        }

        /// <summary>
        /// 求陣列中n個元素的排列
        /// </summary>
        /// <param name="t">所求陣列</param>
        /// <param name="n">元素個數</param>
        /// <returns>陣列中n個元素的排列</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>
        /// 求陣列中n個元素的組合
        /// </summary>
        /// <param name="t">所求陣列</param>
        /// <param name="n">元素個數</param>
        /// <returns>陣列中n個元素的組合的範型</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;
        }
    }

    public class MainClass
    {
        static void Main()
        {
            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);
            //求組合
            List<int[]> lst_Combination = Algorithms.PermutationAndCombination<int>.GetCombination(arr, 3);

            Console.WriteLine(lst_Combination.Count  );
            for (int i = 0; i < lst_Combination.Count ; i++)
            {
                int[] result = lst_Combination[i];
                for (int j = 0; j < result.Length ; j++)
                {
                    Console.Write ("----" + result[j]);
                }
                Console.WriteLine();
            }
       
            Console.ReadKey();

        }
    }
}