QueueOnList

#include <assert.h>
#include <iostream>

struct Node {
  int data;
  Node* next;

  Node(int value) : data(value), next(nullptr) {}
};

// Очередь целых чисел.
class Queue {
 public:
  Queue(int size) : head(nullptr), tail(nullptr) {}
  ~Queue() {
    for (Node* c = tail; c != nullptr;) {
      Node* next = c->next;
      delete c;
      c = next;
    }
  }

  // Добавление и извлечение элемента из очереди.
  void enqueue(int a) {
    if (tail == nullptr) {
      tail = head = new Node(a);
    } else {
      head->next = new Node(a);
      head = head->next;
    }
  }

  int dequeue() {
    assert(tail != 0);
    Node* next = tail->next;
    delete tail;
    tail = next;
    if (tail == nullptr) head = nullptr;
  }

  // Проверка на пустоту.
  bool empty() const { return tail == nullptr; }

 private:
  Node* head;
  Node* tail;
};