#include <list>
template<typename T>
class DoubleArray {
  struct NODE {
    T t;
    NODE* next;
  };
  int size_a;
  int size_b;
  NODE* header_a;
  NODE* header_a_cur;
  NODE* header_b;
  NODE* header_b_cur;
  int trigger;
public:
  DoubleArray() : size_a(0), size_b(0), trigger(0), header_a(0), header_a_cur(0), header_b(0), header_b_cur(0) {
  }
  int push(T t);
  std::list<T>& fetch(std::list<T>& list);
};
template<typename T>
int DoubleArray<T>::push(T t) {
  NODE *n = new NODE;
  n->t = t;
  n->next = 0;
  if (size_a == 0 && trigger == 0) {
    header_a = n;
    header_a_cur = n;
    size_a++;
  } else if (size_b == 0 && trigger == 1) {
    header_b = n;
    header_b_cur = n;
    size_b++;
  } else {
    switch (trigger) {
      case 0:
        header_a_cur->next = n;
        header_a_cur = n;
        size_a++;
        break;
      case 1:
        header_b_cur->next = n;
        header_b_cur = n;
        size_b++;
        break;
    }
  }
}
template<typename T>
std::list<T>& DoubleArray<T>::fetch(std::list<T>& list) {
  switch (trigger) {
    case 0:
      if (header_a != 0) {
        // change b
        trigger = 1;
        // fetch a
        NODE* temp = header_a;
        while (temp) {
          list.push_back(temp->t);
          temp = temp->next;
        }
        // delete a
        temp = header_a;
        for (int i = 0; i < size_a; ++i) {
          NODE* p = temp;
          temp = temp->next;
          delete p;
        }
        size_a = 0;
        header_a = 0;
        header_a_cur = 0;
      }
      break;
    case 1:
      if (header_b != 0) {
        // change a
        trigger = 0;
        // fetch b
        NODE* temp = header_b;
        // delete b
        while (temp) {
          list.push_back(temp->t);
          temp = temp->next;
        }
        temp = header_b;
        for (int i = 0; i < size_b; ++i) {
          NODE* p = temp;
          temp = temp->next;
          delete p;
        }
        size_b = 0;
        header_b = 0;
        header_b_cur = 0;
      }
      break;
  }
  return list;
}