i used deque. cout got me TLE.using printf save me.

      if(st.front()!=s[i])
        {
            st.emplace_front(s[i]);
        }
        else
            st.pop_front();


for(it=st.end()-1; it>=st.begin(); it--)
    {
        printf("%c",*it);
    }

i say more - it's better don't use stl-containers if it's possible.
char[200001] gives perfect time