使用 java 中的方法交换单链表中的两个节点

Question

package linkedlists;

public class SinglyLinkedList<E> implements Cloneable {
  //---------------- nested Node class ----------------
  /**
   * Node of a singly linked list, which stores a reference to its
   * element and to the subsequent node in the list (or null if this
   * is the last node).
   */
  private static class Node<E> {

    /** The element stored at this node */
    private E element;            // reference to the element stored at this node

    /** A reference to the subsequent node in the list */
    private Node<E> next;         // reference to the subsequent node in the list

    /**
     * Creates a node with the given element and next node.
     *
     * @param e  the element to be stored
     * @param n  reference to a node that should follow the new node
     */
    public Node(E e, Node<E> n) {
      element = e;
      next = n;
    }

    // Accessor methods
    /**
     * Returns the element stored at the node.
     * @return the element stored at the node
     */
    public E getElement() { return element; }

    /**
     * Returns the node that follows this one (or null if no such node).
     * @return the following node
     */
    public Node<E> getNext() { return next; }

    // Modifier methods
    /**
     * Sets the node's next reference to point to Node n.
     * @param n    the node that should follow this one
     */
    public void setNext(Node<E> n) { next = n; }
    
  } //----------- end of nested Node class -----------

  // instance variables of the SinglyLinkedList
  /** The head node of the list */
  private Node<E> head = null;               // head node of the list (or null if empty)

  /** The last node of the list */
  private Node<E> tail = null;               // last node of the list (or null if empty)

  /** Number of nodes in the list */
  private int size = 0;                      // number of nodes in the list

  /** Constructs an initially empty list. */
  public SinglyLinkedList() { }              // constructs an initially empty list

  // access methods
  /**
   * Returns the number of elements in the linked list.
   * @return number of elements in the linked list
   */
  public int size() { return size; }

  /**
   * Tests whether the linked list is empty.
   * @return true if the linked list is empty, false otherwise
   */
  public boolean isEmpty() { return size == 0; }

  /**
   * Returns (but does not remove) the first element of the list
   * @return element at the front of the list (or null if empty)
   */
  public E first() {             // returns (but does not remove) the first element
    if (isEmpty()) return null;
    return head.getElement();
  }

  /**
   * Returns (but does not remove) the last element of the list.
   * @return element at the end of the list (or null if empty)
   */
  public E last() {              // returns (but does not remove) the last element
    if (isEmpty()) return null;
    return tail.getElement();
  }

  // update methods
  /**
   * Adds an element to the front of the list.
   * @param e  the new element to add
   */
  public void addFirst(E e) {                // adds element e to the front of the list
    head = new Node<>(e, head);              // create and link a new node
    if (size == 0)
      tail = head;                           // special case: new node becomes tail also
    size++;
  }

  /**
   * Adds an element to the end of the list.
   * @param e  the new element to add
   */
  public void addLast(E e) {                 // adds element e to the end of the list
    Node<E> newest = new Node<>(e, null);    // node will eventually be the tail
    if (isEmpty())
      head = newest;                         // special case: previously empty list
    else
      tail.setNext(newest);                  // new node after existing tail
    tail = newest;                           // new node becomes the tail
    size++;
  }

  /**
   * Removes and returns the first element of the list.
   * @return the removed element (or null if empty)
   */
  public E removeFirst() {                   // removes and returns the first element
    if (isEmpty()) return null;              // nothing to remove
    E answer = head.getElement();
    head = head.getNext();                   // will become null if list had only one node
    size--;
    if (size == 0)
      tail = null;                           // special case as list is now empty
    return answer;
  }

  @SuppressWarnings({"unchecked"})
  public boolean equals(Object o) {
    if (o == null) return false;
    if (getClass() != o.getClass()) return false;
    SinglyLinkedList other = (SinglyLinkedList) o;   // use nonparameterized type
    if (size != other.size) return false;
    Node walkA = head;                               // traverse the primary list
    Node walkB = other.head;                         // traverse the secondary list
    while (walkA != null) {
      if (!walkA.getElement().equals(walkB.getElement())) return false; //mismatch
      walkA = walkA.getNext();
      walkB = walkB.getNext();
    }
    return true;   // if we reach this, everything matched successfully
  }

  @SuppressWarnings({"unchecked"})
  public SinglyLinkedList<E> clone() throws CloneNotSupportedException {
    // always use inherited Object.clone() to create the initial copy
    SinglyLinkedList<E> other = (SinglyLinkedList<E>) super.clone(); // safe cast
    if (size > 0) {                    // we need independent chain of nodes
      other.head = new Node<>(head.getElement(), null);
      Node<E> walk = head.getNext();      // walk through remainder of original list
      Node<E> otherTail = other.head;     // remember most recently created node
      while (walk != null) {              // make a new node storing same element
        Node<E> newest = new Node<>(walk.getElement(), null);
        otherTail.setNext(newest);     // link previous node to this one
        otherTail = newest;
        walk = walk.getNext();
      }
    }
    return other;
  }

  public int hashCode() {
    int h = 0;
    for (Node walk=head; walk != null; walk = walk.getNext()) {
      h ^= walk.getElement().hashCode();      // bitwise exclusive-or with element's code
      h = (h << 5) | (h >>> 27);              // 5-bit cyclic shift of composite code
    }
    return h;
  }

  /**
   * Produces a string representation of the contents of the list.
   * This exists for debugging purposes only.
   */
  public String toString() {
    StringBuilder sb = new StringBuilder("(");
    Node<E> walk = head;
    while (walk != null) {
      sb.append(walk.getElement());
      if (walk != tail)
        sb.append(", ");
      walk = walk.getNext();
    }
    sb.append(")");
    return sb.toString();
  }
  
  public void swapNodes(Node<E> num1, Node<E> num2) {
      
      Node<E> num1Prev = this.head;
      Node<E> num2Prev = this.head;
      
      if (num1 == num2 ) 
          return ;
      
      while((num1Prev != null)&&(num1Prev.getNext() != num1)){
          
          num1Prev = num1Prev.getNext();
      }
      
      while((num2Prev != null)&&(num2Prev.getNext() != num2)){
          
          num2Prev = num2Prev.getNext();
      }
      
      if(num2Prev == num1) {
          
          num1.setNext(num2.getNext());
          num2.setNext(num1);
          num1Prev.setNext(num2);
      }
      else if(num1Prev == num2) {
          num2.setNext(num1.getNext());
          num1.setNext(num2);
          num2Prev.setNext(num1);
      }
      else {
          Node<E> tmp = num1.getNext();
          num1.setNext(num2.getNext());
          num2.setNext(tmp);
          
          num1Prev.setNext(num2);
          num2Prev.setNext(num1);
      }
      
      
  }
  
  
  
  //main method
  public static void main(String[] args)
  {
      
      SinglyLinkedList<String> list = new SinglyLinkedList<String>();
      list.addFirst("MSP");
      list.addLast("ATL");
      list.addLast("BOS");
      //
      list.addFirst("LAX");
      System.out.println(list);
      //
      
      SinglyLinkedList<String> swap = new SinglyLinkedList<String>();
      swap.addFirst("1");
      swap.addLast("2");
      swap.addLast("3");
      swap.addLast("4");
      swap.addLast("5");
      
      System.out.println("Original list: " + swap);  

      swap.swapNodes("2","5");
      
      System.out.println("After Swapping list: " + swap);  

  }
  
}

Task: In this exercise, you will add a method swapNodes to SinglyLinkedList class from week 2 lecture examples. This method should swap two nodes node1 and node2 (and not just their contents) given references only to node1 and node2. The new method should check if node1 and node2 are the same nodes, etc. Write the main method to test the swapNodes method. Hint: You may need to traverse the list.

I made this method

public void swapNodes(Node<E> num1, Node<E> num2) {
      
      Node<E> num1Prev = this.head;
      Node<E> num2Prev = this.head;
      
      if (num1 == num2 ) 
          return ;
      
      while((num1Prev != null)&&(num1Prev.getNext() != num1)){
          
          num1Prev = num1Prev.getNext();
      }
      
      while((num2Prev != null)&&(num2Prev.getNext() != num2)){
          
          num2Prev = num2Prev.getNext();
      }
      
      if(num2Prev == num1) {
          
          num1.setNext(num2.getNext());
          num2.setNext(num1);
          num1Prev.setNext(num2);
      }
      else if(num1Prev == num2) {
          num2.setNext(num1.getNext());
          num1.setNext(num2);
          num2Prev.setNext(num1);
      }
      else {
          Node<E> tmp = num1.getNext();
          num1.setNext(num2.getNext());
          num2.setNext(tmp);
          
          num1Prev.setNext(num2);
          num2Prev.setNext(num1);
      }
      
      
  }
  

And then, created an instance to check if it's work, but it is showing me an error on here swap.swapNodes("2", "5"); Does anyone know what is the problem? Thank you

Answer 1

It's throwing error, because in your swapNodes function, you expect two parameters of type Node<E> passed, but you are passing E (String) . So you have to change signature of the function or pass Node<E> that you added to your List.

Here's how you could do your swapNodes function with parameters of type E :

public void swapNodes(E element1, E element2) {
    if (element1 == element2 || element1 == null || element2 == null || isEmpty()) return;
    Node<E> num1 = findNode(element1);
    Node<E> num2 = findNode(element2);
    if (num1 == null || num2 == null) return;
    Node<E> num1Prev = this.head;
    Node<E> num2Prev = this.head;
  
    while(num1Prev != null && num1Prev.getNext() != num1){
        num1Prev = num1Prev.getNext();
    }
    while(num2Prev != null && num2Prev.getNext() != num2){
        num2Prev = num2Prev.getNext();
    }
    if (num1Prev.getNext() == null || num2Prev.getNext() == null) return;
  
    if (num2Prev.equals(num1)) {
        num1.setNext(num2.getNext());
        num2.setNext(num1);
        num1Prev.setNext(num2;
    } else if (num1Prev.equals(num2)) {
        num2.setNext(num1.getNext());
        num1.setNext(num2);
        num2Prev.setNext(num1);
    } else {
        Node<E> tmp = num1.getNext();
        num1.setNext(num2.getNext());
        num2.setNext(tmp);
        num1Prev.setNext(num2);
        num2Prev.setNext(num1);
    }
}

The findNode function could look like this:

public Node<E> findNode(E element) {
    if (isEmpty()) return null;
    Node<E> node = this.head;
    while(node != null) {
        if (node.getElement() == element) return node;
    }
    return null;
}