cows and oxes

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace cows_and_oxes
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        int k = 4;
        int i;
        const int n = 9001;
        string[] list = new string[n];
        int[] linkfrom = new int[n];
        int[] linkto = new int[n];
        int first = 0;
        int p = n;
        int m = 0;

        public void comp(string s1, string s2, ref int a1, ref int a2)
        {
            for (int i = 0; i < s1.Length; i++)
                for (int j = 0; j < s2.Length; j++)
                {
                    if (s1[i] == s2[j])
                    {
                        if (i == j)
                            a1++;
                        else
                            a2++;
                    }
                }
            if (s1 == s2)
            {
                a1 = k;
                a2 = 0;
            }
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            int r;
            for (int i = 0; i < n; i++)
                list[i] = "";
            for (i = 1000; i < 10000; i++)
            {

                string s = i.ToString();
                //while (s.Length < k)
                //{
                //    s = '0' + s;
                //}
                r = i - 1000;
                list[r] = s;
                linkto[r] = r + 1;
                linkfrom[r] = r - 1;
            }
            linkto[n - 1] = -1;
            //for (i = 0; i < n; i++)
            //    richTextBox1.AppendText(list[i] + '\n');

            richTextBox1.AppendText(list[0] + '\n');
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string input = textBox1.Text;
            //richTextBox2.Text = string.Empty;
            string[] el = input.Split(' ', '\n');
            int cows = int.Parse(el[0]);
            int oxes = int.Parse(el[1]);
            int a1, a2;
            int t;
            if (m != -1 && p > 1)
            {
                i = first;
                while (i != -1)
                {
                    t = linkto[i];
                    a1 = 0;
                    a2 = 0;
                    comp(list[m], list[i], ref a1, ref a2);
                    if (a1 != cows || a2 != oxes)
                    {
                        if (i == first)
                        {
                            int b;
                            if (linkto[i] != -1)
                            {
                                b = linkto[linkto[i]];
                                first = linkto[i];
                                linkfrom[linkto[i]] = linkfrom[i];
                                linkto[i] = b;
                            }
                           // else
                                //linkfrom[linkto[i]] = linkfrom[i];
                        }
                        else
                            if (linkto[i] == -1)
                        {
                            linkto[linkfrom[i]] = linkto[i];
                        }
                        else
                        {
                            linkto[linkfrom[i]] = linkto[i];
                            linkfrom[linkto[i]] = linkfrom[i];
                        }
                        p--;
                    }
                    i = t;
                }
                m = linkto[m];
                if (m != -1)
                    richTextBox1.AppendText(list[m] + '\n');
                textBox1.Text = string.Empty;
            }
            else
            {
                richTextBox1.AppendText("Ваше число: " + list[first]);
            }
        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {

        }
    }
}