Sorry. I asked but now I got the answer and I don't know how to delete this post. My answer is
    long i;
    char c,t[200001];

    i=0;
    t[0]=0;
    while(std::cin.get(c))
    {
         if(c!=t[i])
         {
              i++;t[i]=c;

         }else{i--;};
    }

Edited by author 25.04.2011 20:32