Recursive Linked List Operations

Modify the linked list class presented in this chapter by adding sort() and reverse() methods. The reverse method reverses the order of the elements in the list and the sort method rearranges the elements in the list so they are sorted in alphabetical order. The class should use recurssion to implement the sort and reverse operations. Extend the graphical interface in the LinkedList1Demo class to support sort and reverse commands, and use it to test the new methods.

Here is LinkedList1 code:

/** The LinkedList1 class implements a Linked list. */

class LinkedList1 { /** The Node class stores a list element and a reference to the next node. */ private class Node { String value; Node next; /** Constructor. @param val The element to store in the node. @param n The reference to the successor node. */ Node(String val, Node n) { value = val; next = n; } /** Constructor. @param val The element to store in the node. */ Node(String val) { // Call the other (sister) constructor. this(val, null); } } private Node first; // list head private Node last; // last element in list /** Constructor. */ public LinkedList1() { first = null; last = null; } /** The isEmpty method checks to see if the list is empty. @return true if list is empty, false otherwise. */ public boolean isEmpty() { return first == null; } /** The size method returns the length of the list. @return The number of elements in the list. */ public int size() { int count = 0; Node p = first; while (p != null) { // There is an element at p count ++; p = p.next; } return count; } /** The add method adds an element to the end of the list. @param e The value to add to the end of the list. */ public void add(String e) { if (isEmpty()) { first = new Node(e); last = first; } else { // Add to end of existing list last.next = new Node(e); last = last.next; } } /** The add method adds an element at a position. @param e The element to add to the list. @param index The position at which to add the element. @exception IndexOutOfBoundsException When index is out of bounds. */ public void add(int index, String e) { if (index < 0 || index > size()) { String message = String.valueOf(index); throw new IndexOutOfBoundsException(message); } // Index is at least 0 if (index == 0) { // New element goes at beginning first = new Node(e, first); if (last == null) last = first; return; } // Set a reference pred to point to the node that // will be the predecessor of the new node Node pred = first; for (int k = 1; k <= index - 1; k++) { pred = pred.next; } // Splice in a node containing the new element pred.next = new Node(e, pred.next); // Is there a new last element ? if (pred.next.next == null) last = pred.next; } /** The toString method computes the string representation of the list. @return The string form of the list. */ public String toString() { StringBuilder strBuilder = new StringBuilder(); // Use p to walk down the linked list Node p = first; while (p != null) { strBuilder.append(p.value + " "); p = p.next; } return strBuilder.toString(); } /** The remove method removes the element at an index. @param index The index of the element to remove. @return The element removed. @exception IndexOutOfBoundsException When index is out of bounds. */ public String remove(int index) { if (index < 0 || index >= size()) { String message = String.valueOf(index); throw new IndexOutOfBoundsException(message); } String element; // The element to return if (index == 0) { // Removal of first item in the list element = first.value; first = first.next; if (first == null) last = null; } else { // To remove an element other than the first, // find the predecessor of the element to // be removed. Node pred = first; // Move pred forward index - 1 times for (int k = 1; k <= index -1; k++) pred = pred.next; // Store the value to return element = pred.next.value; // Route link around the node to be removed pred.next = pred.next.next; // Check if pred is now last if (pred.next == null) last = pred; } return element; } /** The remove method removes an element. @param element The element to remove. @return true if the remove succeeded, false otherwise. */ public boolean remove(String element) { if (isEmpty()) return false; if (element.equals(first.value)) { // Removal of first item in the list first = first.next; if (first == null) last = null; return true; } // Find the predecessor of the element to remove Node pred = first; while (pred.next != null && !pred.next.value.equals(element)) { pred = pred.next; }

// pred.next == null OR pred.next.value is element if (pred.next == null) return false; // pred.next.value is element pred.next = pred.next.next; // Check if pred is now last if (pred.next == null) last = pred; return true; } public static void main(String [] args) { LinkedList1 ll = new LinkedList1(); ll.add("Amy"); ll.add("Bob"); ll.add(0, "Al"); ll.add(2, "Beth"); ll.add(4, "Carol"); System.out.println("The members of the list are:"); System.out.print(ll); } }