Tree Recovery

The Problem

Given the preorder and inorder traversal of a tree write the postorder traversal of the tree.

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Collections;

namespace ConsoleApplication1

{

    class Program

    {

        static string answer = "";

        static void Main(string[] args)

        {

            string preord = "12345678";

            string inord = "32541876";

            former(preord, inord, false);

            while(s.Count>0)

            {

                answer = answer + s.Pop();

            }

            Console.WriteLine(answer);

        }

        static Stack s = new Stack();

        static void former(string preord, string inord, bool control)       

        {

            if (inord.Length == 1 || inord.Length < 1)

            {

                if (inord.Length == 1)

                {

                    answer = answer + inord;

                    if (control == true)

                    {

                        answer = answer + s.Pop();

                    }

                    return;

                }

                else

                {

                    if (control == true)

                    {

                        answer = answer + s.Pop();

                    }

                    return;

                }

            }

            else

            {

                int i=0, j=0;

                for (i = 0; i < preord.Length; i++)

                {

                    bool flag = false;

                    for (j = 0; j < inord.Length; j++)

                    {

                        if (preord[i] == inord[j])

                        {

                            flag = true;

                            break;

                        }

                    }

                    if(flag == true)

                    {break;}

                }

                string s3 = inord;

                string s4 = preord;

                //Left sub-tree in the inorder traversal.

                string s2 = inord.Remove(j, inord.Length - j);

                //Left sub-tree in the preorder traversal.

                string s1 = preord.Substring(i + 1, s2.Length);

                //Right sub-tree in the inorder traversal.

                string s5 = s3.Remove(0, j + 1);

                int index = s4.IndexOf(s1, 0);

                //Right sub-tree in the preorder traversal.

                string s6 = s4.Remove(index, s1.Length);

                //Push the root into the stack.

                s.Push(s6[0]);

                //Right sub-tree without the root in the preorder traversal.

                s6 = s6.Remove(0, 1);

                control = false;

                former(s1, s2, control);

                control = true;

                former(s6, s5, control);

            }

        }

    }

}