问题
I'm writing code for an assignment that requires a method which reverses elements in a LinkedList, given the portion of the list to reverse. For example, if the user enters 3 the method would reverse the first 3 elements in the array. I've written code for it but instead of reversing the code it simply replaces the 2nd element with the element present in the first index. My only issue seems to be the reverseFirstSome method. I'm not asking you to write the code for me, but any pointer in the right direction would be appreciated. Here is my full code for the class:
import java.util.NoSuchElementException;
public class LinkedList
{
//nested class to represent a node
private class Node
{
public Object data;
public Node next;
}
//only instance variable that points to the first node.
private Node first;
// Constructs an empty linked list.
public LinkedList()
{
first = null;
}
// Returns the first element in the linked list.
public Object getFirst()
{
if (first == null)
{
NoSuchElementException ex
= new NoSuchElementException();
throw ex;
}
else
return first.data;
}
// Removes the first element in the linked list.
public Object removeFirst()
{
if (first == null)
{
NoSuchElementException ex = new NoSuchElementException();
throw ex;
}
else
{
Object element = first.data;
first = first.next; //change the reference since it's removed.
return element;
}
}
// Adds an element to the front of the linked list.
public void addFirst(Object element)
{
//create a new node
Node newNode = new Node();
newNode.data = element;
newNode.next = first;
//change the first reference to the new node.
first = newNode;
}
// Returns an iterator for iterating through this list.
public ListIterator listIterator()
{
return new LinkedListIterator();
}
public String toString() {
LinkedListIterator iterator = new LinkedListIterator();
String result = "{ ";
while (iterator.hasNext())
result += (iterator.next() + " ");
result += "}\n";
return result;
}
public int size(){
LinkedListIterator iterator = new LinkedListIterator();
int a = 0;
while (iterator.hasNext()){
iterator.next();
a++;
}
return a;
}
public void addElement(Object obj, int index){
if (index < 0 || index > size() ){
IndexOutOfBoundsException ex = new IndexOutOfBoundsException();
throw ex;
}
LinkedListIterator iterator = new LinkedListIterator();
for(int i = 0; i < index; i++){
if (iterator.hasNext()){
iterator.next();
}else{
NoSuchElementException ex = new NoSuchElementException();
throw ex;
}
}
if (iterator.hasNext()){
Object a = iterator.next();
iterator.set(obj);
while(iterator.hasNext()){
Object b = iterator.next();
iterator.set(a);
a = b;
}
iterator.add(a);
}
else
iterator.add(obj);
}
public Object getElement(int index)
{
LinkedListIterator it = new LinkedListIterator();
for(int i = 0; i < index; i++)
{it.next();}
return it.next();
}
public Object removeElement(int index)
{
if(index<0)
{NoSuchElementException ex = new NoSuchElementException();
throw ex;}
if(index>size()-1)
{NoSuchElementException ex = new NoSuchElementException();
throw ex;}
Object result = null;
LinkedListIterator it = new LinkedListIterator();
result = getElement(index);
if(index<size()-1)
{
for(int i = 0; i<index+1; i++)
it.next();
while(index<size()-2)
{
it.set(getElement(index+1));
it.next();
index++;
}
it.remove();
}
else
{
for(int i = 0; i<index+1; i++)
it.next();
it.remove();
}
return result;
}
public String findSmallest() {
LinkedListIterator iterator = new LinkedListIterator();
if (!iterator.hasNext()){
return null;
}
String smallest = (String) getFirst();
while (iterator.hasNext()) {
if (smallest.compareToIgnoreCase((String) iterator.next()) > 1) {
smallest = (String) iterator.next();
}
}
return smallest;
}
public void searchAndReplace(Object first, Object second){
LinkedListIterator iterator = new LinkedListIterator();
while(iterator.hasNext()){
if(iterator.next().equals(first)){
iterator.set(second);
}
}
}
public void searchAndRemove(Object toBeRemoved){
LinkedListIterator iterator = new LinkedListIterator();
int a = 0;
while(iterator.hasNext()){
if(iterator.next().equals(toBeRemoved)){
removeElement(a);
}
a++;
}
}
public void reverseFirstSome(int howMany){
LinkedListIterator it = new LinkedListIterator();
if(size() > 1){
int top = howMany - 1;
int bot = 0;
it.next();
do{
LinkedListIterator it1 = new LinkedListIterator();
Object one = getElement(bot);
Object two = getElement(top);
it.set(two);
it.next();
for(int i = 0; i < top + 1; i++){
it1.next();
it1.set(one);
bot++;
top--;
}
}while(top > (size()/2) - 1);
}
}
//nested class to define its iterator
private class LinkedListIterator implements ListIterator
{
private Node position; //current position
private Node previous; //it is used for remove() method
// Constructs an iterator that points to the front
// of the linked list.
public LinkedListIterator()
{
position = null;
previous = null;
}
// Tests if there is an element after the iterator position.
public boolean hasNext()
{
if (position == null) //not traversed yet
{
if (first != null)
return true;
else
return false;
}
else
{
if (position.next != null)
return true;
else
return false;
}
}
// Moves the iterator past the next element, and returns
// the traversed element's data.
public Object next()
{
if (!hasNext())
{
NoSuchElementException ex = new NoSuchElementException();
throw ex;
}
else
{
previous = position; // Remember for remove
if (position == null)
position = first;
else
position = position.next;
return position.data;
}
}
// Adds an element after the iterator position
// and moves the iterator past the inserted element.
public void add(Object element)
{
if (position == null) //never traversed yet
{
addFirst(element);
position = first;
}
else
{
//making a new node to add
Node newNode = new Node();
newNode.data = element;
newNode.next = position.next;
//change the link to insert the new node
position.next = newNode;
//move the position forward to the new node
position = newNode;
}
//this means that we cannot call remove() right after add()
previous = position;
}
// Removes the last traversed element. This method may
// only be called after a call to the next() method.
public void remove()
{
if (previous == position) //not after next() is called
{
IllegalStateException ex = new IllegalStateException();
throw ex;
}
else
{
if (position == first)
{
removeFirst();
}
else
{
previous.next = position.next; //removing
}
//stepping back
//this also means that remove() cannot be called twice in a row.
position = previous;
}
}
// Sets the last traversed element to a different value.
public void set(Object element)
{
if (position == null)
{
NoSuchElementException ex = new NoSuchElementException();
throw ex;
}
else
position.data = element;
}
} //end of LinkedListIterator class
} //end of LinkedList class
回答1:
I can think of an implementation of appending to standard logic of reversing a list.
If you pass index greater than the number of elements in the list, then it simply reverses the entire list.
If you pass 0 or 1, the list will be unaffected.
public boolean reverseTillIndex(int index) {
int count = 0;
if (index == 0) {
return false;
}
Node endCountNode = head;
while (count++ < index && endCountNode != null) {
endCountNode = endCountNode.next;
}
count = 0;
// standard reverse a list code
Node current = head;
Node h2 = null;
while (current != null && count++ < index) {
head = current.next;
current.next = h2;
h2 = current;
current = head;
}
head = h2;
while (h2.next != null) {
h2 = h2.next;
}
h2.next = endCountNode;
return true;
}
回答2:
Here is an implementation of reverseSomeMethod() inside your LinkedList class which I believe to be logically correct. It uses recursion to reverse the order of the list. In the base case, it assigns the new head to the current tail, and at each step it links the list in the reverse direction.
public void reverseSomeMethod (Node node) {
if (node == null || node.next == null) {
return;
}
reverseSomeMethodHelper(node);
// assign the new tail (the previous head) of the list to point to NULL
node.next = null;
}
private void reverseSomeMethodHelper(Node node) {
// base case: assign the new HEAD to old tail, link in reverse, and return
if (node.next.next == null) {
setFirst(node.next);
node.next.next = node;
return;
}
// otherwise traverse deeper in the list
reverseSomeMethodHelper(node.next);
// link in reverse order and return
node.next.next = node;
}
Note that you may have to tweak my code to work with your current implementation. For example, I didn't make much of getters and setters, because your original code did not have them.
来源：https://stackoverflow.com/questions/43269638/issues-with-reversing-objects-in-a-linkedlist