Suppose we have a simple graph like this:
It was easy to find a path from start node to end node with depth-first search, but I got stuck while trying to do the same thing with breadth-first search. My code is as follows:
public List<String> getPathBreadth(String name1, String name2) {
Node node1 = getNode(name1);
Node node2 = getNode(name2);
if (node1 == null || node2 == null) {
return null;
}
return getPathBreadth(node1, node2, new HashSet<String>(), new LinkedList<Node>());
}
private List<String> getPathBreadth(Node start, Node destination, HashSet<String> visited, Queue<Node> queue) {
List<String> path = new ArrayList<String>();
if (start == destination) {
path.add(start.name);
return path;
}
visited.add(start.name);
queue.offer(start);
while (queue.size() > 0) {
Node cur = queue.poll();
for (String friend : cur.friends) {
if (visited.contains(friend)) {
continue;
}
visited.add(friend);
if (getNode(friend) == destination) {
path.add(friend); // I got the final node, I could also add cur, but how do I get the previous nodes along the path
return path;
}
queue.offer(getNode(friend));
}
}
return path;
}
Suppose we want to go from John to Linda, so I wish to return something like [Linda, Robert, John] or [Linda, Patrica, John], but the best I can do for now is get the last and second to the last nodes. In this case they are Linda and Robert. How do I get all the previous nodes along the path?
To make the code usable please add Node definition, and tree (test data).
(see mcve ).
I think the problem is here :
if (getNode(friend) == destination) {
path.add(friend);
return path;
}
The only node you add to the path in the last one. Try:
path.add(friend);
if (getNode(friend) == destination) {
return path; //or better: break;
}
Unfortunately I can't run and check it.
A side note:
visited.add(friend) Returns true if this set did not already contain friend
so:
if (visited.contains(friend)) {
continue;
}
visited.add(friend);
can be replaced with
if (! visited.add(friend)) {
continue;
}
Hello i'm trying to delete a node based on a key. I'm learning dictionary implementation and decided to implement it from scratch to fully understand the concept. i successfully was able to add and return the value using 2 node references a head and tail pointer. But i'm having difficulty using the key to delete a node from the list.
Below is my code to delete from the list
public V remove(K key) {
V result = null;
if(!this.isEmpty()&&head==tail){ //if head and tail are only nodes in the list
if(tail.getKey().equals(key)){
result = tail.getValue();
head=null;
tail = null;
count--; //decrements the count in the list
}
}
else {
boolean found = false;
Node current = head;
Node previous = null;
while(current!=null&&!found){
if(current.getKey().equals(key)){
previous = current;
result = current.getValue();
previous.setNextNode(current.getNextNode());
previous.setNextNode(null);
count--;
found = true;
}
current = current.getNextNode();
}
}
return result;
}
when i enter the desired key to be deleted. It deletes all the keys after the desired key to be deleted.
PS it's not a double linked list. I just created a tail node to access the last node in the list
It seems that you are getting bogged down with updating the list. This code simplifies your algorithm:
// this method returns the head of the new list with the item possibly removed
public Node remove (K key) {
V result = null;
if (this.isEmpty()) {
return null; // return null for empty list
} else if (head.getKey().equals(key)) {
return head.getNextNode(); // return head if head matches
} else {
Node current = head;
Node next = head.getNextNode();
while (next != null) { // walk down the list and search
if (next.getKey().equals(key)) {
current.setNextNode(next.getNextNode());
break;
}
current = next; // advance list pointers
next = next.getNextNode();
}
return head; // will return head for list with 1 element
}
}
You got the general idea, except for 2 things: previous = current should be done outside the if block so that it always gets assigned before moving current forward, and previous.setNextNode(null) should be removed as it undoes the preceding line.
Also, you need to make a special case when the first node in the list matches the key so you can reassign the head.
while(current!=null&&!found){
if(current.getKey().equals(key)){
result = current.getValue();
previous.setNextNode(current.getNextNode());
count--;
found = true;
}
previous = current;
current = current.getNextNode();
}
You have two errors:
You set previous = current;, as the first statement, meaning previous and current are always the same.
You should remove this line:
previous.setNextNode(null); As you do it right after assigning the new next node.
I have JXTreeTable which I can filter. When I filter a keyword a new model is set. It works ok for me.
Now I want to expand all the filtered results. For that I save the position where the matching node was found. (It cant be a leaf.)
For that positions I create a List of TreePaths with this method:
public TreePath getPath(TreeNode node) {
List<TreeNode> list = new ArrayList<TreeNode>();
while (node != null) {
list.add(node);
node = node.getParent();
}
Collections.reverse(list);
return new TreePath(list.toArray());
}
Now I iterate over this list and call expandPath (after the new model was set)
Where getTreePaths() is a list of TreePaths which I created with the method before.
for (TreePath elem : f.getTreePaths()) {
tree.expandPath(elem);
tree.scrollPathToVisible(elem);
}
But it has no effect, only the root is expanded and all the children are collapsed.
In the TreePath the last element is no leaf. In a System.out in this loop I get for all:
-1
true
true
true
System.out.println(tree.getRowForPath(elem)); System.out.println(f.isPathValid(elem,tree.getTreeTableModel()));
System.out.println(tree.isVisible(elem));
System.out.println(tree.isExpanded(elem));
The -1 for getRowForPath is maybe the problem?
isPathValid():
public boolean isPathValid(TreePath path,TreeTableModel model) {
if (path.getPathCount() == 0) {
return model.getRoot().equals(path.getPathComponent(0));
}
for (int x = 1; x < path.getPathCount(); x++) {
if (model.getIndexOfChild(path.getPathComponent(x - 1),
path.getPathComponent(x)) == -1) {
return false;
}
}
return true;
}
I know it is an old post, but I'll drop here my findings after spending an hour in a similar problem.
The method "isPathValid" only validates if the sequence is correct by checking if the child (x) belongs to the parent (x - 1), but it doesn't validate if the root is the same as the table model (unless the path count is zero).
When filtering, you usually trigger a new root to the table model, so if you captured the TreePaths "before" updating the model (and triggering a new root), the table won't be able to find the path, although the path itself is valid.
It should work with a few of changes:
public **List** getPath(TreeNode node) {
List<TreeNode> list = new ArrayList<TreeNode>();
**while (node != model.getRoot()) {**
list.add(node);
node = node.getParent();
}
Collections.reverse(list);
**return list;**
}
**for (List elem : f.getTreePaths()) {**
elem.add(0, model.getRoot());
tree.expandPath(new TreePath(elem.toArray());
tree.scrollPathToVisible(elem);
}
Im trying to write the methods to "union" which can be described: if A, B, C are sets, has the form C = A.union(B). Union returns a Set that contains all the elements in set A and B, but only list duplicates once.
My idea for this method is to traverse set A and add all of its elements to the union set, then traverse set B, if an element of set B is already present in the union set, then do not insert it to result, otherwise insert all to the union set.
This is complicated for a beginner like me since I want to include all 3 lists into the method (which I get a bunch of errors). I already wrote some methods in my SLinkedList class to check and add element but the parameters take an object from a Node
/** Singly linked list .*/
public class SLinkedList {
protected Node head; // head node of the list
protected int size; // number of nodes in the list
/** Default constructor that creates an empty list */
public SLinkedList() {
head = new Node(null, null); // create a dummy head
size = 0;
// add last
public void addLast(Object data) {
Node cur = head;
// find last node
while (cur.getNext() != null) {
cur = cur.getNext();
}
// cur refers to the last node
cur.setNext(new Node(data, null));
size++;
}
// contain method to check existing elements
public boolean contain (Object target) {
boolean status = false;
Node cursor;
for (cursor = head; cursor != null; cursor = cursor.getNext()) {
if (target.equals(cursor.getElement())) {
status = true;
}
}
return status;
}
public SLinkedList union (SLinkedList secondSet) {
SLinkedList unionSet = new SLinkedList();
secondSet = new SLinkedList();
Node cursor;
for(cursor = head.getNext(); cursor != null; cursor = cursor.getNext()) {
unionSet.addLast(cursor.getElement());
// traverse secondSet, if an element is existed in either set A or union
// set, skip, else add to union set
}
}
return unionSet;
}
}
Node class
/** Node of a singly linked list of strings. */
public class Node {
private Object element; // we assume elements are character strings
private Node next;
/** Creates a node with the given element and next node. */
public Node(Object o, Node n) {
element = o;
next = n;
}
/** Returns the element of this node. */
public Object getElement() { return element; }
/** Returns the next node of this node. */
public Node getNext() { return next; }
// Modifier methods:
/** Sets the element of this node. */
public void setElement(Object newElem) { element = newElem; }
/** Sets the next node of this node. */
public void setNext(Node newNext) { next = newNext; }
}
*update*
The question is if there is a second list involved public SLinkedList union (SLinkedList secondSet) , what syntax should i use to traverse set B and check if an element of set B is already present in result then do not insert it to result, otherwise insert. Do I need to creat a node for set B and traverse it?, there may be a compare method to compare the 2 sets outside of the union method?
Please help. Thanks all.
SLinkedList unionSet = null; // need a new SLinkedList() here
Node cursor;
for(cursor = head.getNext(); cursor != null; cursor = cursor.getNext()) {
unionSet.addLast(cursor.getElement()); // NPE because unionSet is null
}
I thought I had understanding of this in my previous question about linked lists, but I was terribly mistaken, I'm just as lost as I was when I initially posted.
I realize that I'm technically asking two questions, but hopefully getting at least one should make the other easy (assuming they are just reverse of each other).
I have 3 classes already given to me, they are:
SLinkedList.java
package chapter3.linkedList;
public class SLinkedList<V> {
// instance variables. Add the tail reference.
protected Node<V> head, tail;
protected long size;
// methods, empty list constructor first
public SLinkedList () {
head = null;
tail = null;
size = 0;
} // end constructor of a SLinkedList
// method to add nodes to the list. Storage space for the node
// is already allocated in the calling method
public void addFirst (Node<V> node) {
// set the tail only if this is the very first node
if (tail == null)
tail = node;
node.setNext (head); // make next of the new node refer to the head
head = node; // give head a new value
// change our size
size++;
} // end method addFirst
// addAfter - add new node after current node, checking to see if we are at the tail
public void addAfter (Node<V>currentNode, Node<V>newNode) {
if (currentNode == tail)
tail = newNode;
newNode.setNext (currentNode.getNext ());
currentNode.setNext (newNode);
// change our size
size++;
} // end method addAfter
// addLast - add new node after the tail node. Adapted from Code Fragment 3.15, p. 118.
// Mike Qualls
public void addLast (Node<V> node) {
node.setNext (null);
tail.setNext (node);
tail = node;
size++;
} // end method addLast
// methods to remove nodes from the list. (Unfortunately, with a single linked list
// there is no way to remove last. Need a previous reference to do that. (See
// Double Linked Lists and the code below.)
public Node<V> removeFirst () {
if (head == null)
System.err.println("Error: Attempt to remove from an empty list");
// save the one to return
Node<V> temp = head;
// do reference manipulation
head = head.getNext ();
temp.setNext(null);
size--;
return temp;
} // end method removeFirst
// remove the node at the end of the list. tail refers to this node, but
// since the list is single linked, there is no way to refer to the node
// before the tail node. Need to traverse the list.
public Node<V> removeLast () {
// // declare local variables/objects
Node<V> nodeBefore;
Node<V> nodeToRemove;
// make sure we have something to remove
if (size == 0)
System.err.println("Error: Attempt to remove fron an empty list");
// traverse through the list, getting a reference to the node before
// the trailer. Since there is no previous reference.
nodeBefore = getFirst ();
// potential error ?? See an analysis and drawing that indicates the number of iterations
// 9/21/10. size - 2 to account for the head and tail nodes. We want to refer to the one before the
// tail.
for (int count = 0; count < size - 2; count++)
nodeBefore = nodeBefore.getNext ();
// save the last node
nodeToRemove = tail;
// now, do the pointer manipulation
nodeBefore.setNext (null);
tail = nodeBefore;
size--;
return nodeToRemove;
} // end method removeLast
// method remove. Remove a known node from the list. No need to search or return a value. This method
// makes use of a 'before' reference in order to allow list manipulation.
public void remove (Node<V> nodeToRemove) {
// declare local variables/references
Node<V> nodeBefore, currentNode;
// make sure we have something to remove
if (size == 0)
System.err.println("Error: Attempt to remove fron an empty list");
// starting at the beginning check for removal
currentNode = getFirst ();
if (currentNode == nodeToRemove)
removeFirst ();
currentNode = getLast ();
if (currentNode == nodeToRemove)
removeLast ();
// we've already check two nodes, check the rest
if (size - 2 > 0) {
nodeBefore = getFirst ();
currentNode = getFirst ().getNext ();
for (int count = 0; count < size - 2; count++) {
if (currentNode == nodeToRemove) {
// remove current node
nodeBefore.setNext (currentNode.getNext ());
size--;
break;
} // end if node found
// change references
nodeBefore = currentNode;
currentNode = currentNode.getNext ();
} // end loop to process elements
} // end if size - 2 > 0
} // end method remove
// the gets to return the head and/or tail nodes and size of the list
public Node<V> getFirst () { return head; }
public Node<V> getLast () { return tail; }
public long getSize () { return size; }
} // end class SLinkedList
There's also Node.java
package chapter3.linkedList;
public class Node<V>
{
// instance variables
private V element;
private Node<V> next;
// methods, constructor first
public Node ()
{
this (null, null); // call the constructor with two args
} // end no argument constructor
public Node (V element, Node<V> next)
{
this.element = element;
this.next = next;
} // end constructor with arguments
// set/get methods
public V getElement ()
{
return element;
}
public Node<V> getNext ()
{
return next;
}
public void setElement (V element)
{
this.element = element;
}
public void setNext (Node<V> next)
{
this.next = next;
}
} // end class Node
and finally GameEntry.java
package Project_1;
public class GameEntry
{
protected String name; // name of the person earning this score
protected int score; // the score value
/** Constructor to create a game entry */
public GameEntry(String name, int score)
{
this.name = name;
this.score = score;
}
/** Retrieves the name field */
public String getName()
{
return name;
}
/** Retrieves the score field */
public int getScore()
{
return score;
}
/** Returns a string representation of this entry */
public String toString()
{
return name + ", " + score + "\n";
}
}
EDIT POINT
I created a driver called Scores.java, in it so far all I have is **I have added what I THINK i need for the classes, I'm probably wrong though:
package Project_1;
import chapter3.linkedList.*;
import java.util.*;
/** Class for storing high scores in an array in non-decreasing order. */
public class Scores
{
//add function
public SLinkedList<GameEntry> add(GameEntry rank, SLinkedList<GameEntry> scores)
{
Node<GameEntry> currentNode = scores.getFirst();
Node<GameEntry> nextNode = null;
Node<GameEntry> previousNode = null;
Node<GameEntry> newNode = new Node<GameEntry>();
newNode.setElement(rank);
if(scores.getSize() == 0)
{
scores.addFirst(newNode);
}
else
{
while(currentNode != null)
{
nextNode = currentNode.getNext();
if(nextNode == null)
{
scores.addLast(newNode);
}
else
{
scores.addAfter(currentNode, newNode);
break;
}
previousNode = currentNode;
currentNode = currentNode.getNext();
}
}
return scores;
}
//remove function
public void remove(int i)
{
}
//print function
/*gameenter printing;
printing=node.Getelement; //pseudo code for making it work right
print(printing.getscore)
print(print.getname)
*/
public void print(SLinkedList<GameEntry> scores)
{
Node<GameEntry> currentNode = scores.getFirst();
GameEntry currentEntry = currentNode.getElement();
System.out.printf("[");
for(int i = 0; i < scores.getSize(); i++)
{
System.out.printf(", %s", currentEntry.toString());
currentNode = currentNode.getNext();
currentEntry = currentNode.getElement();
}
System.out.println("]");
}
}
I have my test driver called ScoresTest.java, that I have pretty much filled out:
package Project_1;
import chapter3.linkedList.SLinkedList;
public class ScoresTest {
/**
* #param args
*/
public static void main(String[] args)
{
SLinkedList<GameEntry> highScores = new SLinkedList<GameEntry>(); //Linked List for Game Entry
GameEntry entry;
Scores rank = new Scores();
entry = new GameEntry("Flanders", 681);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Krusty", 324);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Otto", 438);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Bart", 875);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Homer", 12);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Lisa", 506);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Maggie", 980);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Apoo", 648);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Smithers", 150);
highScores = rank.add(entry, highScores);
entry = new GameEntry("Burns", 152);
highScores = rank.add(entry, highScores);
System.out.println("The Original High Scores");
rank.print(highScores);
entry = new GameEntry("Moe", 895);
highScores = rank.add(entry, highScores);
System.out.println("Scores after adding Moe");
rank.print(highScores);
//highScores = rank.remove(4);
System.out.println("Scores after removing Apoo");
rank.print(highScores);
}
}
That's entirely finished, pretty sure I have nothing left to add for it.
I'm not looking for someone to answer it for me, but I have no clue where to start or how to make the add or remove function, in any way. This is an intermediate course, the book does nothing for explaining linked lists (go ahead and look for yourself if you don't believe me, text is called Datastructures and Algorithms in Java, 5th edition). It shows how to do such with an array quite easily...which works perfectly for a linked list, but apparently the teacher does not want us doing it this way, so sadly I am now utterly lost on how to do this.
I've tried looking at other peoples answers on here, and google, and so far nothing has clicked or made any sense at all, I just can't grasp how it works, and the teacher's explanation and example was only to draw boxes on the board, I've never seen a sort, add, or remove function coded for a linked list...can't know what I've not been taught or can't locate.
Any help is greatly appreciated, and thank you in advance!
EDIT
I looked at the import java.util.*; and the commands within it for linked lists, they seem painfully easy. to remove I'd just use list.sublist(i, i).clear(); and the value I wish to remove is removed, super easy, it seems to be just trying to utilize the slinkedlist.java and node.java, I just can't seem to follow them in any way shape or form. I believe the teacher did indeed write them, and I've tried asking for his assistance, stayed 2 hours after class trying to get any understanding from him, and as you can see it did not help much at all. Thank you again for the assistance!
EDIT
I also apologize if this seems like it is vague, but I don't have a specific point where my confusion seems linked, I understand linked lists if we're talking about the java.util.linkedList;, but as far as using what I've been given in this circumstance, I can't follow the logic at all, leaving me quite lost and unsure of where to begin.
In pseudo-code (please note I am not including bound checking etc, simply the logic)
To add a node to the front of the list:
newNode->nextNode = startNode
startNode = newNode
To add to a specific index:
index = 0
currentNode = startNode
// find the node in the list. here you will need to do all kinds of bound checking
while index is less than position
currentNode = currentNode.nextNode // move your node pointer to the position
increment index
// so here we basically insert the new node into the list. what needs to happen is
// to NOT break the list by forgetting the node after the current node. this is why
// we first set the new nodes' next one, to the current nodes' (the one already in
// the list) next node. this way, we still have all the information we need. then,
// when we set the current nodes' next node to the new node, we essentially "break"
// the link and "repair" it by adding the new link.
newNode.nextNode = currentNode.nextNode // some more bound checking required
currentNode.nextNode = newNode
To remove from a specific index:
index = 0
delNode = startNode
// find the node in the list. here you will need to do all kinds of bound checking
while index is less than (position - 1)
delNode = delNode.nextNode // move your node pointer to the position
increment index
delNode.nextNode = delNode.nextNode.nextNode
// that's it. by setting the node's (before the one you whish to delete)
// next node to the node AFTER the one you want to delete, you basically
// "skip" over that node. since it is no longer referenced, the garbage
// collector will take care of the rest. if you wish to return that node
// you can do it quite easily by remembering it.
storeNode = delNode.nextNode // some more bound checking required
delNode.nextNode = delNode.nextNode.nextNode // some more bound checking required
// now you still have a reference to the deleted node in storeNode
UPDATE
OK, so if I understand correctly, you need to create a linked list that stores scores in a ascending order. As far as I can see, the entire linked list has been implemented for you, you simply need to use the classes provided, and add the logic in Scores.java to keep the list sorted.
First off, I see your nodes are not comparable. If you are at all allowed to change the source given to you, I would suggest having them implement Comparable<Node> and also override the equals(Object o) so that you have logic to compare them. Two nodes can contain the same element, but that does not mean that they are equal.
Please note the change in the method signatures!
//add function
public void add(Node<GameEntry> score) {
// adding is where you now want to keep everything sorted. so I highly
// recommend that you implement `Comparable` as I mentioned above. if not,
// you have to put the logic in here.
Node<GameEntry> currentNode = highScored.getFirst();
Node<GameEntry> prevNode = null;
// if the list is empty, or the new node must go in before the head,
// simply add it as the head.
if (highScores.size() == 0 || score.compareTo(currentNode) < 0) {
highScores.addFirst(score);
}
// search for the position of the new node. while the node has a higher score
// than the current node, we need to continue on so we can place it in the
// correct place.
while (currentNode != null && currentNode.compareTo(score) > 0) {
prevNode = currentNode;
currentNode = currentNode.getNext();
}
// if the currentNode is null, it means it is the highest score, so
// we can simply add it to the end
if (currentNode == null) {
highScores.addLast(score);
} else {
// otherwise just add it after the correct node
highScores.addAfter(prevNode, score);
}
}
//remove function
public void remove(Node<GameEntry> score) {
// removing an element should be as described above. if you keep
// your list sorted during the ADD method, removing any element
// should not break the order.
// find the element - removal from a linked list is O(n),
// since we need to know what the element BEFORE the one
// is that you want to remove. assuming you have implemented
// the equals method to check equality of nodes:
Node<GameEntry> currentNode = highScores.getFirst();
Node<GameEntry> prevNode = null;
while (currentNode != null && !currentNode.equals(score)) {
prevNode = currentNode;
currentNode = currentNode.getNext();
}
// if currentNode is null, the node we wanted to remove was not
// in the list.
if (currentNode == null) {
System.out.println("Node not found");
return;
}
// now, we need to check if there is a node after the one we want
// to remove.
if (prevNode.getNext().getNext() != null) {
// if there is, we follow the logic from the pseudo code
prevNode.setNext(prev.getNext().getNext());
} else {
// if not, we only need to remove the last entry (since the
// one we want to remove is the last one)
highScores.removeLast();
}
}
IMPORTANT
Please just double check the logic here. I did it really quickly without an IDE as I'm not at my development computer at the moment. If anyone finds any issues, please leave a comment and I'll fix it.
If this is not exactly what you asked (your question is a bit vague), let me know.
UPDATE 2
Read up on Comparators here, here and here.