I am having an issue organizing a linked list alphabetically. I am reading the names in from a text file and storing them into a linked list. The problem I am having is how to sort them alphabetically. If anybody could point me in the right direction that would be amazing. The idea is to get the value of the first 3 letters in each name and compare them to the first 3 in the next name. But where would I compare the letters together?
Here is the LinkedListNode class:
public class LinkedListNode
{
private String data;
private LinkedListNode next;
public LinkedListNode(String data)
{
this.data = data;
this.next = null;
}
public String getData()
{
return data;
}
public LinkedListNode getNext()
{
return next;
}
public void setNext(LinkedListNode n)
{
next = n;
}
}
Here is the LinkedList file with the main method:
import java.io.*;
import java.util.Scanner;
public class LinkedList {
public LinkedListNode head;
String fname;
public static void main(String[] args) throws FileNotFoundException{
Scanner scan = new Scanner(new File("Names.txt"));
LinkedList l = new LinkedList();
int i = 1;
while(scan.hasNext()) {
String s = scan.nextLine();
l.insertBack(s);
i++;
}
System.out.print(l.showList());
}
public LinkedList() {
this.head = null;
}
public void insertBack(String data){
if(head == null){
head = new LinkedListNode(data);
}else{
LinkedListNode newNode = new LinkedListNode(data);
LinkedListNode current = head;
while(current.getNext() != null){
current = current.getNext();
}
current.setNext(newNode);
}
}
public String showList(){
int i = 0, j;
String retStr = "List nodes:\n";
LinkedListNode current = head;
while(current != null){
i++;
retStr += "Node " + i + ": " + current.getData() + "\n";
current = current.getNext();
}
return retStr;
}
}
Some pseudo code for you:
OUTER:
for word in file
node = head
while node.next
if word > node.word
node.next
else
Node temp = new Node(word)
temp.next = word.next
node.next = temp
continue OUTER
node.next = new Node(word)
This is an as-you-go insertion sort. After every insert the file will be sorted. Or you could use other sorting algorithms after you read all of the data
if it's if word > node.word this part you're having trouble with, the String#compareTo method will be useful
Try using Collections.sort(list)
Also, for comparing, you can use compareTo function under Comparable Interface
To do easy comparisons, your nodes should implement Comparable. The base Java libraries tend to rely upon this for easy sorting.
The Comaprable interface will require you to implement compareTo (see below).
public int <LinkedListNode> compareTo(LinkedListNode n){
//Case insensitively compare the first 3 characters of the two nodes
String myHead = data.substring(0,3).toLowerCase();
String comparableHead = n.data.substring(0,3).toLowerCase();
return (myHead.compareTo(comparableHead));
}
If you use a standard List structure like, ArrayList, the Collections.sort(list) will be able to use this method to order your list.
And here's an insertion sort based "insert" function for your runTime, using this comparable.
public void insert(String data){
LinkedListNode newNode = new LinkedListNode(data);
if(head == null){
head = newNode;
}else{
LinkedListNode current = head;
LinkedListNode prev;
//This is missing some key edge cases, but it inserts properly in the general case. You'll have to add to it to handle things like running off the list, or this needing to be inserted before the head.
while(current.getNext() != null){
if(current.compareTo(newNode)<0){
newNode.setNext(current);
prev.setNext(newNode);
break;
}
prev = current;
current = current.getNext();
}
}
}
Related
This is a code that reverses linked list using stack. I got everything right from creating a linked list and getting input from the user, but I got some logic wrong. Iam trying to reverse linked list using stack, but it is only reversing two number if say suppose I give in three numbers. How do I get it right?
import java.util.*;
class LinkedList{
Node head;
static class Node{`
int data;
Node next;
Node(int d){
this.data = d;
next = null;
}
}
public static LinkedList insert(LinkedList list, int data){
Node new_node = new Node(data);
if(list.head == null){
list.head = new_node;
}
else{
Node last = list.head;
while(last.next != null){
last = last.next;
}
last.next = new_node;
}
return list;
}
public static LinkedList reverse(LinkedList list){
Stack<Integer> stack = new Stack<Integer>();
Node current = list.head;
while(current.next != null){
stack.push(current.data);
current = current.next;
}
while(!stack.isEmpty()){
System.out.println("The reversed list is: " + stack.pop());
}
return list;
}
}
public class Main
{
public static void main(String[] args) {
LinkedList l = new LinkedList();
Scanner sc = new Scanner(System.in);
int length = sc.nextInt();
for(int i=1; i<=length; i++){
int number = sc.nextInt();
l.insert(l,number);
}
l.reverse(l);
}
}
The problem lies with your reverse function. While reading the linked list your while loop logic is skipping the last item.
Correct function is:
public static LinkedList reverse(LinkedList list){
Stack<Integer> stack = new Stack<Integer>();
Node current = list.head;
while(current != null){ // instead of while(current.next != null)
System.out.println("Pushed Data:"+current.data);
stack.push(current.data);
current = current.next;
}
System.out.println("The reversed list is: ");
while(!stack.isEmpty()){
System.out.println(stack.pop());
}
return list;
}
Everything looks good except the line while(current.next != null){ as here you check for the next node instead of the current node. So you need to change this line to while(current != null){
I implemented a solution to reverse a linked list in java that I found online. But it is not working in my code for some reason.
When I print the list it only prints the first node. I know the print method works because it prints the whole thing when I don't try to reverse.
Where did I go wrong in this code?
public class LinkedLists {
public static void main(String[] args) {
LinkedList list = new LinkedList();
list.addLast(10);
list.addLast(20);
list.addLast(30);
list.reverseList();
list.print();
}
public static class LinkedList{
private class Node{
private int value;
private Node next;
}
public Node first;
public Node last;
public void addLast(int item){
Node node = new Node();
node.value = item;
if(first == null) {
first = node;
last = node;
} else {
last.next = node;
last = node;
}
}
private Node reverse(Node head, Node newHead) {
//base case: when first = last you return
if(head == null) {
return newHead;
}
Node temp = head.next;
head.next = newHead; //this will initially be null
newHead = head;
head = temp;
return reverse(head, newHead);
}
public Node reverseList() {
return reverse(first, null);
}
public void print(){
Node current = first;
while (current != null){
System.out.print(current.value + " ");
current = current.next;
}
}
} //class ends
}
Although reverse returns the correct reference for the new head, the initial call of reverseList -- in the main program -- ignores this returned reference.
Your reverseList method should better not return anything, but instead update the first and last members:
public void reverseList() {
last = first;
first = reverse(first, null);
}
Can someone provide the possible ways to print a Linkedlist in reverse in Java.
A way I understand would be to Recursively reach the end of list, And then start printing from the back and come to front recursively.
Please share the possible ways.
I am using a Node having next and previous.
A Solution I figured is below. But here I need to create a variable each time entering in the recursive loop. That's bad :(
public void reversePrinting(int count){
if(count==0){ //to assign the root node to current only once
current=root;
count++;
}
else{ //moving current node to subsequent nodes
current=current.nextNode;
}
int x= current.data;
if(current.nextNode==null){
System.out.println(x);
return;
}
reversePrinting(count);
System.out.println(x);
}
try this, it is able reverse a linkedlist
public class DoReverse{
private Node head;
private static class Node {
private int value;
private Node next;
Node(int value) {
this.value = value;
}
}
public void addToTheLast(Node node) {
if (head == null) {
head = node;
}
else {
Node temp = head;
while (temp.next != null)
temp = temp.next;
temp.next = node;
}
}
public void printList(Node head) {
Node temp = head;
while (temp != null) {
System.out.format("%d ", temp.value);
temp = temp.next;
}
System.out.println();
}
public static Node reverseList(Node head){
Node prev = null;
Node current = head;
Node next = null;
while(current != null){
next = current.next;
current.next = prev;
prev = current;
current = next;
}
head = prev;
return head;
}
public static void main(String[] args) {
DoReverse list = new DoReverse();
// Creating a linked list
Node head = new Node(5);
list.addToTheLast(head);
list.addToTheLast(new Node(6));
list.addToTheLast(new Node(7));
list.addToTheLast(new Node(1));
list.addToTheLast(new Node(2));
list.addToTheLast(new Node(10));
System.out.println("Before Reversing :");
list.printList(head);
Node reverseHead= list.reverseList(head);
System.out.println("After Reversing :");
list.printList(reverseHead);
}
}
Why not copy the list so that it is reversed:
Reversing a linked list in Java, recursively
And then loop the copy of the list like your normally would?
I have a question for combining two linkedlist. Basically, I want to append one linkedlist to the other linkedlist.
Here is my solution. Is there a more efficient way to do it without looping the first linkedlist? Any suggestion would be appreciated.
static Node connect(LinkedList list1, LinkedList list2) {
Node original = list1.first;
Node previous = null;
Node current = list1.first;
while (current != null) {
previous = current;
current = current.next;
}
previous.next = list2.first;
return original;
}
Use list1.addAll(list2) to append list2 at the end of list1.
For linked lists, linkedList.addAll(otherlist) seems to be a very poor choice.
the java api version of linkedList.addAll begins:
public boolean addAll(int index, Collection<? extends E> c) {
checkPositionIndex(index);
Object[] a = c.toArray();
so even when you have 2 linked lists, the second one gets converted to an array, then re-constituted into individual elements. This is worse than just merging 2 arrays.
I guess this is your own linked list implementation? With only a pointer to next element, the only way to append at the end is to walk all the elements of the first list.
However, you could store a pointer to the last element to make this operation run in constant time (just remember to update the last element of the new list to be the last element of the added list).
The best way is to append the second list to the first list.
1. Create a Node Class.
2. Create New LinkedList Class.
public class LinkedList<T> {
public Node<T> head = null;
public LinkedList() {}
public void addNode(T data){
if(head == null) {
head = new Node<T>(data);
} else {
Node<T> curr = head;
while(curr.getNext() != null) {
curr = curr.getNext();
}
curr.setNext(new Node<T>(data));
}
}
public void appendList(LinkedList<T> linkedList) {
if(linkedList.head == null) {
return;
} else {
Node<T> curr = linkedList.head;
while(curr != null) {
addNode((T) curr.getData());
curr = curr.getNext();
}
}
}
}
3. In the Main function or whereever you want this append to happen, do it like this.
LinkedList<Integer> n = new LinkedListNode().new LinkedList<Integer>();
n.addNode(23);
n.addNode(41);
LinkedList<Integer> n1 = new LinkedListNode().new LinkedList<Integer>();
n1.addNode(50);
n1.addNode(34);
n.appendList(n1);
I like doing this way so that there isn't any need for you to pass both these and loop again in the first LinkedList.
Hope that helps
My Total Code:
NOTE: WITHOUT USING JAVA API
class Node {
Node next;
int data;
Node(int d){
data = d;
next = null;
}
}
public class OddEvenList {
Node head;
public void push(int new_data){
Node new_node = new Node(new_data);
new_node.next = head;
head = new_node;
}
Node reverse(Node head){
Node prev = null;
Node next = null;
Node curr = head;
while(curr != null){
next = curr.next;
curr.next = prev;
prev = curr;
curr = next;
}
head = prev;
return head;
}
Node merge(Node head1, Node head2){
Node curr_odd = head1;
Node curr_even = head2;
Node prev = null;
while(curr_odd != null){
prev = curr_odd;
curr_odd = curr_odd.next;
}
prev.next = curr_even;
return head1;
}
public void print(Node head){
Node tnode = head;
while(tnode != null){
System.out.print(tnode.data + " -> ");
tnode = tnode.next;
}
System.out.println("Null");
}
public static void main(String[] args) {
// TODO Auto-generated method stub
OddEvenList odd = new OddEvenList();
OddEvenList even = new OddEvenList();
OddEvenList merge = new OddEvenList();
odd.push(1);
odd.push(3);
odd.push(5);
odd.push(7);
odd.push(9);
System.out.println("Odd List: ");
odd.print(odd.head);
System.out.println("Even List: ");
even.push(0);
even.push(2);
even.push(4);
even.push(6);
even.push(8);
even.print(even.head);
System.out.println("After Revrse: --------------------");
Node node_odd =odd.reverse(odd.head);
Node node_even = even.reverse(even.head);
System.out.println("Odd List: ");
odd.print(node_odd);
System.out.println("Even List: ");
even.print(node_even);
System.out.println("Meged: --------------");
Node merged = merge.merge(node_odd, node_even);
merge.print(merged);
}
}
I am implementing my own linked list in Java. The node class merely has a string field called "name" and a node called "link". Right now I have a test driver class that only inserts several names sequentially. Now, I am trying to write a sorting method to order the nodes alphabetically, but am having a bit of trouble with it. I found this pseudocode of a bubblesort from someone else's post and tried to implement it, but it doesn't fully sort the entries. I'm not really quite sure why. Any suggestions are appreciated!
private void sort()
{
//Enter loop only if there are elements in list
boolean swapped = (head != null);
// Only continue loop if a swap is made
while (swapped)
{
swapped = false;
// Maintain pointers
Node curr = head;
Node next = curr.link;
Node prev = null;
// Cannot swap last element with its next
while (next != null)
{
// swap if items in wrong order
if (curr.name.compareTo(next.name) < 0)
{
// notify loop to do one more pass
swapped = true;
// swap elements (swapping head in special case
if (curr == head)
{
head = next;
Node temp = next.link;
next.link = curr;
curr.link = temp;
curr = head;
}
else
{
prev.link = curr.link;
curr.link = next.link;
next.link = curr;
curr = next;
}
}
// move to next element
prev = curr;
curr = curr.link;
next = curr.link;
}
}
}
I spent some minutes eyeballing your code for errors but found none.
I'd say until someone smarter or more hard working comes along you should try debugging this on your own. If you have an IDE like Eclipse you can single-step through the code while watching the variables' values; if not, you can insert print statements in a few places and hand-check what you see with what you expected.
UPDATE I
I copied your code and tested it. Apart from the fact that it sorts in descending order (which may not be what you intended) it worked perfectly for a sample of 0, 1 and 10 random nodes. So where's the problem?
UPDATE II
Still guessing what could be meant by "it doesn't fully sort the entries." It's possible that you're expecting lexicographic sorting (i.e. 'a' before 'B'), and that's not coming out as planned for words with mixed upper/lower case. The solution in this case is to use the String method compareToIgnoreCase(String str).
This may not be the solution you're looking for, but it's nice and simple. Maybe you're lazy like I am.
Since your nodes contain only a single item of data, you don't really need to re-shuffle your nodes; you could simply exchange the values on the nodes while leaving the list's structure itself undisturbed.
That way, you're free to implement Bubble Sort quite simply.
you should use the sorting procedures supplied by the language.
try this tutorial.
Basically, you need your element class to implement java.lang.Comparable, in which you will just delegate to obj.name.compareTo(other.name)
you can then use Collections.sort(yourCollection)
alternatively you can create a java.util.Comparator that knows how to compare your objects
To obtain good performance you can use Merge Sort.
Its time complexity is O(n*log(n)) and can be implemented without memory overhead for lists.
Bubble sort is not good sorting approach. You can read the What is a bubble sort good for? for details.
This may be a little too late. I would build the list by inserting everything in order to begin with because sorting a linked list is not fun.
I'm positive your teacher or professor doesn't want you using java's native library. However that being said, there is no real fast way to resort this list.
You could read all the nodes in the order that they are in and store them into an array. Sort the array and then relink the nodes back up. I think the Big-Oh complexity of this would be O(n^2) so in reality a bubble sort with a linked list is sufficient
I have done merge sort on the singly linked list and below is the code.
public class SortLinkedList {
public static Node sortLinkedList(Node node) {
if (node == null || node.next == null) {
return node;
}
Node fast = node;
Node mid = node;
Node midPrev = node;
while (fast != null && fast.next != null) {
fast = fast.next.next;
midPrev = mid;
mid = mid.next;
}
midPrev.next = null;
Node node1 = sortLinkedList(node);
Node node2 = sortLinkedList(mid);
Node result = mergeTwoSortedLinkedLists(node1, node2);
return result;
}
public static Node mergeTwoSortedLinkedLists(Node node1, Node node2) {
if (null == node1 && node2 != null) {
return node2;
} else if (null == node2 && node1 != null) {
return node1;
} else if (null == node1 && null == node2) {
return null;
} else {
Node result = node1.data <= node2.data ? node1 : node2;
Node prev1 = null;
while (node1 != null && node2 != null) {
if (node1.data <= node2.data) {
prev1 = node1;
node1 = node1.next;
} else {
Node next2 = node2.next;
node2.next = node1;
if (prev1 != null) {
prev1.next = node2;
}
node1 = node2;
node2 = next2;
}
}
if (node1 == null && node2 != null) {
prev1.next = node2;
}
return result;
}
}
public static void traverseNode(Node node) {
while (node != null) {
System.out.print(node + " ");
node = node.next;
}
System.out.println();
}
public static void main(String[] args) {
MyLinkedList ll1 = new MyLinkedList();
ll1.insertAtEnd(10);
ll1.insertAtEnd(2);
ll1.insertAtEnd(20);
ll1.insertAtEnd(4);
ll1.insertAtEnd(9);
ll1.insertAtEnd(7);
ll1.insertAtEnd(15);
ll1.insertAtEnd(-3);
System.out.print("list: ");
ll1.traverse();
System.out.println();
traverseNode(sortLinkedList(ll1.start));
}
}
The Node class:
public class Node {
int data;
Node next;
public Node() {
data = 0;
next = null;
}
public Node(int data) {
this.data = data;
}
public int getData() {
return this.data;
}
public Node getNext() {
return this.next;
}
public void setData(int data) {
this.data = data;
}
public void setNext(Node next) {
this.next = next;
}
#Override
public String toString() {
return "[ " + data + " ]";
}
}
The MyLinkedList class:
public class MyLinkedList {
Node start;
public void insertAtEnd(int data) {
Node newNode = new Node(data);
if (start == null) {
start = newNode;
return;
}
Node traverse = start;
while (traverse.getNext() != null) {
traverse = traverse.getNext();
}
traverse.setNext(newNode);
}
public void traverse() {
if (start == null)
System.out.println("List is empty");
else {
Node tempNode = start;
do {
System.out.print(tempNode.getData() + " ");
tempNode = tempNode.getNext();
} while (tempNode != null);
System.out.println();
}
}
}