using System.Collections.Generic;

using System.Linq;

using System.Text;

public string TransgformString(string s)

{

    var n = s.Length;

    var stack = new Stack<char>();

    for (var i = 0; i < n; i++)

    {

        if (stack.Count == 0)

        {

            stack.Push(s[i]);

        }

        else

        {

            if (stack.Peek() == 'A' && s[i] == 'B')

            {

                stack.Pop();

            }

            else if (stack.Peek() == 'B' && s[i] == 'A')

            {

                stack.Pop();

            }

            else if (stack.Peek() == 'C' && s[i] == 'D')

            {

                stack.Pop();

            }

            else if (stack.Peek() == 'D' && s[i] == 'C')

            {

                stack.Pop();

            }

            else

            {

                stack.Push(s[i]);

            }

        }

    }

​

    var result = new StringBuilder();

    while (stack.Count > 0)

    {

        result.Append(stack.Pop());

    }

​

    return new string(result.ToString().Reverse().ToArray());

}