C# 求素数和，整数数组顺序排列，递归函数 全排列

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace MathDemo
{
    class Program
    {
        static void Main(string[] args)
        {
            //取1－1000的素数和。素数只能被1和本身相除。
            /*
            const int number = 1000;
            int count = 0;
            for (int i = 1; i < number; i++)
            {
                int k = 0;
                for (int j = 2; j < i; j++)
                {
                    if (i % j == 0)
                    {
                        k++;
                        break;
                    }
                }
                if (k == 0)
                {
                    count += i;
                }
            }
            Console.WriteLine(count);
            Console.ReadLine();
             */
            //对一个不规则的整数数组排列，第n个元素和后续的元素相比，元素换位。
            //int[] a = { 5, 2, 7, 9, 1 };
            //for (int i = 0; i < a.Length; i++)
            //{
            //    for (int j = i+1; j<a.Length; j++)
            //    {
            //        if (a[i] > a[j])
            //        {
            //            int temp = a[i];
            //            a[i] = a[j];
            //            a[j] = temp;
            //        }
            //    }
            //    Console.WriteLine(a[i]);
            //}
            //Console.ReadLine();
            //全排列
            char[] array = ("ABCDEF").ToCharArray();
            List<string> lists = new List<string>();
            AppendNextElement(array,lists,1);
            if (lists.Count > 0)
            {
                for (int i = 0; i < lists.Count; i++)
                {
                    Console.WriteLine(lists[i]);
                }
                Console.ReadLine();
            }     
        }
        public static void AppendNextElement(char[] element, List<string> lists,int AppendTime)
        {  
                //把所有的char 数组中的元素全放入list,第一次附加
                if (AppendTime == 1)
                {
                    for (int i = 0; i < element.Length; i++)
                    {
                        if (!lists.Contains(element[i].ToString()))
                        {
                            lists.Add(element[i].ToString());
                        }
                    }
                }
                //第1次以后，所有的元素都又会循环附加char数组的元素，但是如果已有list元素中含有即将被附加进来的元素，则不附加，例如("AA","ABA","ACBC")
                else
                {
                    //循环次数已现在有的长度为准
                    int length = lists.Count;
                    for (int j = 0; j < length; j++)
                    {
                        for (int k = 0; k < element.Length; k++)
                        {
                            if (lists[j].Length == 1)
                            {
                                if (lists[j] != element[k].ToString())
                                {
                                    string temp = lists[j] + element[k].ToString();
                                    lists.Add(temp);
                                }  
                            }
                            else
                            {
                                //判断最后一个元素是不是和要附加的元素是否相等，相等则不附加
                                if (lists[j].Substring(lists[j].Length - 1) != element[k].ToString())
                                {
                                    //判断列表中的元素是否包含要附加的元素，有则不附加
                                    if (!lists[j].Contains(element[k].ToString()))
                                    {
                                        string temp = lists[j] + element[k].ToString();
                                        lists.Add(temp);
                                    }
                                }
                            }
                        }
                    }
                    //附加完后考虑到前面生成的元素没有被删除，删除前面的元素。比如数组char A,B
                    //第2次附加完后应该是A,B,AB,BA,则删除元素A和B,数量是数学公式排列p2,1
                    //数组Char A,B,C
                    //第2次附加完后应该是A,B,C,AB,AC,BA,BC,CA,CB则删除元素A,B和C
                    //第3次附加完后应该删除AB,AC,BA,BC,CA,CB
                    if (AppendTime > 1)
                    {
                        lists.RemoveRange(0, getjiechen(element.Length,AppendTime-1));  
                    }
                }    
            //第1次 附加AppendTime = 1
            //第2次 附加AppendTime = 2
            //.......                3
            //.......                4
            //.......                .
            AppendTime++;
            if (AppendTime <= element.Length)
            {
                AppendNextElement(element, lists, AppendTime);
            }
            return;
        }
        //根据char 数组的长度和第几次附加，取得对应的阶乘
        public static int getjiechen(int length,int count)
        {
            int total = 1;
            for (int i = 0; i < count; i++)
            {
                total *= length;
                length--;
            }
            return total;
        }
    }
}