I do not argue, just I want to know.
Thanks.

Because it can be solved using data structure called "stack".

Now I got it :) Thank you very much.

Stack<char> Stack = new Stack<char>();
foreach (char c in Console.ReadLine())
    if (Stack.Count > 0 && Stack.Peek() == c) Stack.Pop(); else Stack.Push(c);
foreach (char c in Stack.Reverse()) Console.Write(c);

Because of this code.

After AC i realized i used stack :)

Cuz' String Or a Char array is a data structure and working with data structures actually means changing adding expanding etc. them :) Hope that'll help you

Yes, and because choosing the right data structure makes the problem solvable, and  choosing the wrong one results in TLE.   I had a brain blockage and couldn't think of stacks here (it's been a long time since I used them).   Trying to solve using, for example, STL vector or dequeue and erase() will result in TLE on the big input cases.    I realized that since very many erase() operations may be performed, delete performance is critically important, so STL list came to the rescue and got AC.