using System;

using System.Text;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        string s= "acaaabbbacdddd"; // "MISSISSIPIE";

        string res = "";

        Stack<char> st = new Stack<char>();

        st.Push(s[0]);

        char prev = s[0];

        for(int i=1;i<s.Length;i++) {

            if(st.Count!=0 && st.Peek() == s[i]){

                prev = st.Peek();

                st.Pop();

            } 

            else if(prev==s[i]) {

            }

            else if(st.Count!=0 && st.Peek() != s[i]) {             

                prev = s[i];

                st.Push(s[i]);

            }           

        }

        while(st.Count!=0){

            res+=st.Pop();

        }

        char[] arr = res.ToCharArray();

        Array.Reverse(arr);

            Console.WriteLine("Result "+ new string(arr));

​

    }

}