text: // MIT License
  
  // Copyright (c) 2018 Yang Le
  
  // Permission is hereby granted, free of charge, to any person obtaining a copy
  // of this software and associated documentation files (the "Software"), to deal
  // in the Software without restriction, including without limitation the rights
  // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  // copies of the Software, and to permit persons to whom the Software is
  // furnished to do so, subject to the following conditions:
  
  // The above copyright notice and this permission notice shall be included in
  // all copies or substantial portions of the Software.
  
  // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  // SOFTWARE.
  
  #pragma once
  
  #include <string>
  #include "linked_list.hpp"
  
  template <typename T>
  class Queue {
   public:
    bool isEmpty() const { return !linked_list_.tail_; }
  
    const T* peek() const {
      if (!linked_list_.head_) {
        return nullptr;
      }
  
      return &linked_list_.head_->value_;
    }
  
    void enqueue(const T& value) { linked_list_.append(value); }
  
    T dequeue() {
      auto removedHead = linked_list_.deleteHead();
      return removedHead ? removedHead->value_ : T();
    }
  
    std::string toString(std::function<std::string(const T&)> callback) const {
      return linked_list_.toString(callback);
    }
  
    std::string toString() const { return linked_list_.toString(); }
  
   private:
    LinkedList<T> linked_list_;
  };
  