void sort() const {
      //bubble sort
      if (!empty() && !onlyOneNode()){
         int N = size();
         bool hasSwapped = false;
         for(int i=0; i<N; i++){
            int j = i;
            for (iterator it = ++begin(); it != end() && j < N; it++){
               // cout << *it << endl;
               // cout <<it._node->_prev->_data<<endl;
               if (*it < (it._node->_prev->_data)){
                  hasSwapped = true;
                  it._node->_next->_prev = it._node->_prev;
                  it._node->_prev->_prev->_next = it._node;

                  DListNode<T>* n_tmp = it._node->_next;
                  it._node->_next = it._node->_prev;
                  it._node->_prev->_next = n_tmp;

                  DListNode<T>* p_tmp = it._node->_prev->_prev;
                  it._node->_prev->_prev = it._node;
                  it._node->_prev = p_tmp;
                  it++;
               }
               j++;
            }
            // cout << "N: " << N << ", i: " << i << ", j: " << j << endl;
            if(!hasSwapped) break;
         }
      }
   }