完整的链表不会打印所有值答案

【问题标题】：Complete Linked List will not Print all Value完整的链表不会打印所有值
【发布时间】：2021-03-15 12:18:41
【问题描述】：

我认为我的 Queue 类有问题，因为我知道使用 Queue 会使用 FIFO 方法，但我想确保当我添加一个元素时，它会添加到 Queue 的末尾。在我的主程序中，我添加了数字 1-4，但是当我想使用 Queue 类中的 toString 方法打印整个队列时，它只会打印出第一个元素 1。我将不胜感激！

谢谢！

 public class QuestionFive
    {
       public static void main(String[] args)
       {
          // creating a queue
          Queue q = new Queue();
    
          // adding numbers 1,2,3 and 4
          q.insert(1);
          q.insert(2);
          q.insert(3);
          q.insert(4);

          System.out.println(q);
       }
    }
    class Queue 
    {
       //Private Data Member
       private Link _head;
       
       //Constructor: A constructor to create an empty Queue,
       public Queue() 
       {
          _head = null;
       }
       
       //Insert Method: A method to insert a given Object into the Queue.
       //Note: We will inserton
       public void insert(Object item) 
       {
          Link add = new Link();
          add.data = item;
          add.next = null;
          
          if(_head == null)
          {
             _head = add;
          }
          else
          {
             for(Link curr = _head; curr.next != null; curr = curr.next)
             {
                curr.next = add;
             }
          }
       }
       
       //Delete Method: A method to delete an Object from the Queue
       public Object delete() 
       {
            if ( _head == null ) return null; 
               
            Link prev = null;    
            Link curr = _head;     
            while (curr.next != null )    
            {      
                prev = curr;      
                curr = curr.next; 
            }     
            if ( prev != null ) 
            {
                prev.next = null; 
            }   
            else
            {       
                _head = null;   
            } 
            return curr.data;
        }
        
        // IsEmpty Method: A method to test for an empty Queue
        public boolean isEmpty() 
        {
          // queue is empty if front is null
          return (_head == null);
        }
        
        //toString Method:
        public String toString()
        {
          String s = "";
          for (Link curr = _head; curr != null; curr = curr.next)
          {
              s = s + " " + curr.data;
          }
          return s;
        }
    }
    
    //Link Class
    class Link 
    {
       public Object data;
       public Link next;
    }

【问题讨论】：

您是否尝试过使用调试器和/或在将项目添加到队列时记录代码的行为？在我看来，您的 insert 方法实际上是在每次添加内容时重新编写队列，因此您最终只得到 _head 条目。
curr.next = add; 属于循环之后，而不是循环内部。
@KevinAnderson 如果 curr.next = add 在循环之外，它不会工作吗？
它不会，除非你在循环之前声明curr并将循环写为for (curr = _head;...。我们的想法是让循环将curr 一直移动到最后一个节点，然后让curr 仍然可用，这样你就可以将新节点放在末尾next = add;，

标签： java linked-list queue tostring implementation

【解决方案1】：

一个更简单的方法是引入一个 tail 以及一个 head，使您的队列成为双端，每次添加项目时都无需遍历整个队列。

class Queue {
       private Link head;
       private Link tail;
       
       public void insert(Object item) {
          Link add = new Link();
          add.data = item;
          add.next = null;
          
          if(head == null)
          {
             head = add;
             tail = add;
          }
          else {
             tail.next = add;
             tail = add;
          }
       }
}

该方法确实应该被称为add，因为它是追加到末尾而不是插入。

【讨论】：

非常感谢您的帮助！有效！非常感谢您的帮助！
@rockyDodgers 如果可行，则将答案标记为已接受。

【解决方案2】：

insert() 方法中的逻辑需要注意。我已将方法的else 部分修改如下，它起作用了：

          else
          {
             for(Link curr = _head; ; curr = curr.next)
             {
                if(curr.next == null){
                    curr.next = add;
                    break;
                }
             }
          }

这是完整的工人阶级：

 public class QuestionFive
    {
       public static void main(String[] args)
       {
          // creating a queue
          Queue q = new Queue();
    
          // adding numbers 1,2,3 and 4
          q.insert(1);
          q.insert(2);
          q.insert(3);
          q.insert(4);

          System.out.println(q);
       }
    }
    class Queue 
    {
       //Private Data Member
       private Link _head;
       
       //Constructor: A constructor to create an empty Queue,
       public Queue() 
       {
          _head = null;
       }
       
       //Insert Method: A method to insert a given Object into the Queue.
       //Note: We will inserton
       public void insert(Object item) 
       {
          Link add = new Link();
          add.data = item;
          add.next = null;
          
          if(_head == null)
          {
             _head = add;
          }
          else
          {
             for(Link curr = _head; ; curr = curr.next)
             {
                if(curr.next == null){
                    curr.next = add;
                    break;
                }
             }
          }
       }
       
       //Delete Method: A method to delete an Object from the Queue
       public Object delete() 
       {
            if ( _head == null ) return null; 
               
            Link prev = null;    
            Link curr = _head;     
            while (curr.next != null )    
            {      
                prev = curr;      
                curr = curr.next; 
            }     
            if ( prev != null ) 
            {
                prev.next = null; 
            }   
            else
            {       
                _head = null;   
            } 
            return curr.data;
        }
        
        // IsEmpty Method: A method to test for an empty Queue
        public boolean isEmpty() 
        {
          // queue is empty if front is null
          return (_head == null);
        }
        
        //toString Method:
        public String toString()
        {
          String s = "";
          for (Link curr = _head; curr != null; curr = curr.next)
          {
              s = s + " " + curr.data;
          }
          return s;
        }
    }
    
    //Link Class
    class Link 
    {
       public Object data;
       public Link next;
    }

【讨论】：

【解决方案3】：

在您的代码中修复（而不是 toString 方法 printList）：

 
public class QuestionFive
    {
       public static void main(String[] args)
       {
          // creating a queue
          Queue q = new Queue();
    
          // adding numbers 1,2,3 and 4
          q.insert(1);
          q.insert(2);
          q.insert(3);
          q.insert(4);
          q.printList();
          System.out.println(q);
       }
    }
    class Queue 
    {
       //Private Data Member
       private Link _head;
       
       //Constructor: A constructor to create an empty Queue,
       public Queue() 
       {
          _head = null;
       }
       
       //Insert Method: A method to insert a given Object into the Queue.
       //Note: We will inserton
       public void insert(Object item) 
       {
          Link add = new Link();
          add.data = item;
          add.next = _head;
          _head = add;
          
       }
       
       //Prints list data
       public void printList() {
           Link currentLink = _head;
           System.out.print("List: ");
           while(currentLink != null) {
               currentLink.printLink();
               currentLink = currentLink.next;
           }
           System.out.println("");
       }
       
       //Delete Method: A method to delete an Object from the Queue
       public Object delete() 
       {
            if ( _head == null ) return null; 
               
            Link prev = null;    
            Link curr = _head;     
            while (curr.next != null )    
            {      
                prev = curr;      
                curr = curr.next; 
            }     
            if ( prev != null ) 
            {
                prev.next = null; 
            }   
            else
            {       
                _head = null;   
            } 
            return curr.data;
        }
        
        // IsEmpty Method: A method to test for an empty Queue
        public boolean isEmpty() 
        {
          // queue is empty if front is null
          return (_head == null);
        }
        
    }
    
    //Link Class
    class Link 
    {
       public Object data;
       public Link next;
       //Print Link data
       public void printLink() {
           System.out.print("{" + data + "}");
       }
    }

【讨论】：