Does java have a skip list implementation

Question

I find ConcurrentSkipListSet in Java Collection Framework, which is backed up with a skip list. But is there a skip list in Java? A set does not work in my use case. I need a indexable list that supports duplicates.

Answer 1

This answer is 3 years late but I hope it will be useful for those wanting a Java skip list from this moment on :)

This solution allows duplicates as you asked. I follow roughly the guide here http://igoro.com/archive/skip-lists-are-fascinating, so the complexities are similar to that, except delete costs O(nlogn) - as I didn't bother using doubly-linked nodes, I imagine doing so would bring delete down to O(logn).

Code comprises of: an interface, the skip list implementing the interface, and the node class. It is also generic.

You can tune the parameter LEVELS for performance, but remember the space-time tradeoff.

import java.util.Random;

interface SkippableList<T extends Comparable<? super T>> {
    int LEVELS = 5;

    boolean delete(T target);
    void print();
    void insert(T data);
    SkipNode<T> search(T data);
}

public class SkipList<T extends Comparable<? super T>> implements SkippableList<T> {

    public static void main(String[] args) {
        SkipList<Integer> sl = new SkipList<>();
        int[] data = {4,2,7,0,9,1,3,7,3,4,5,6,0,2,8};
        for (int i : data) {
            sl.insert(i);
        }

        sl.print();
        sl.search(4);

        sl.delete(9);
        sl.print();

        sl.insert(69);
        sl.print();
        sl.search(69);
    }

    private final SkipNode<T> head = new SkipNode<>(null);
    private final Random rand = new Random();

    @Override
    public void insert(T data) {
        SkipNode<T> SkipNode = new SkipNode<>(data);
        for (int i = 0; i < LEVELS; i++) {
            if (rand.nextInt((int) Math.pow(2, i)) == 0) { //insert with prob = 1/(2^i)
                insert(SkipNode, i);
            }
        }
    }

    @Override
    public boolean delete(T target) {
        System.out.println("Deleting " + target.toString());
        SkipNode<T> victim = search(target, false);
        if (victim == null) return false;
        victim.data = null;

        for (int i = 0; i < LEVELS; i++) {
            head.refreshAfterDelete(i);
        }

        System.out.println();
        return true;
    }

    @Override
    public SkipNode<T> search(T data) {
        return search(data, true);
    }

    @Override
    public void print() {
        for (int i = 0; i < LEVELS; i++) {
            head.print(i);
        }

        System.out.println();
    }

    private void insert(SkipNode<T> SkipNode, int level) {
        head.insert(SkipNode, level);
    }

    private SkipNode<T> search(T data, boolean print) {
        SkipNode<T> result = null;
        for (int i = LEVELS-1; i >= 0; i--) {
            if ((result = head.search(data, i, print)) != null) {
                if (print) {
                    System.out.println("Found " + data.toString() + " at level " + i + ", so stoppped" );
                    System.out.println();
                }

                break;
            }
        }

        return result;
    }

}

class SkipNode<N extends Comparable<? super N>> {

    N data;
    @SuppressWarnings("unchecked")
    SkipNode<N>[] next = (SkipNode<N>[]) new SkipNode[SkippableList.LEVELS];

    SkipNode(N data) {
        this.data = data;
    }

    void refreshAfterDelete(int level) {
        SkipNode<N> current = this.getNext(level);
        while (current != null && current.getNext(level) != null) {
            if (current.getNext(level).data == null) {
                SkipNode<N> successor = current.getNext(level).getNext(level);
                current.setNext(successor, level);
                return;
            }

            current = current.getNext(level);
        }
    }

    void setNext(SkipNode<N> next, int level) {
        this.next[level] = next;
    }

    SkipNode<N> getNext(int level) {
        return this.next[level];
    }

    SkipNode<N> search(N data, int level, boolean print) {
        if (print) {
            System.out.print("Searching for: " + data + " at ");
            print(level);
        }

        SkipNode<N> result = null;
        SkipNode<N> current = this.getNext(level);
        while (current != null && current.data.compareTo(data) < 1) {
            if (current.data.equals(data)) {
                result = current;
                break;
            }

            current = current.getNext(level);
        }

        return result;
    }

    void insert(SkipNode<N> SkipNode, int level) {
        SkipNode<N> current = this.getNext(level);
        if (current == null) {
            this.setNext(SkipNode, level);
            return;
        }

        if (SkipNode.data.compareTo(current.data) < 1) {
            this.setNext(SkipNode, level);
            SkipNode.setNext(current, level);
            return;
        }

        while (current.getNext(level) != null && current.data.compareTo(SkipNode.data) < 1 && 
                current.getNext(level).data.compareTo(SkipNode.data) < 1) {

            current = current.getNext(level);
        }

        SkipNode<N> successor = current.getNext(level);
        current.setNext(SkipNode, level);
        SkipNode.setNext(successor, level);
    }

    void print(int level) {
        System.out.print("level " + level + ": [");
        int length = 0;
        SkipNode<N> current = this.getNext(level);
        while (current != null) {
            length++;
            System.out.print(current.data.toString() + " ");
            current = current.getNext(level);
        }

        System.out.println("], length: " + length);
    }

}

Answer 2

Fixed the bug in the implementation provided by @PoweredByRice. It threw an NPE for cases when the node deleted was the first node. Other updates include renamed variable names and reverse printing the order of the skip list.

import java.util.Random;

interface SkippableList<T extends Comparable<? super T>> {

  int LEVELS = 5;

  boolean delete(T target);
  void print();
  void insert(T data);
  SkipNode<T> search(T data);
}

class SkipNode<N extends Comparable<? super N>> {

  N data;
  @SuppressWarnings("unchecked")
  SkipNode<N>[] next = (SkipNode<N>[]) new SkipNode[SkippableList.LEVELS];

  SkipNode(N data) {
    this.data = data;
  }

  void refreshAfterDelete(int level) {
    SkipNode<N> current = this;
    while (current != null && current.getNext(level) != null) {
      if (current.getNext(level).data == null) {
        SkipNode<N> successor = current.getNext(level).getNext(level);
        current.setNext(successor, level);
        return;
      }

      current = current.getNext(level);
    }
  }

  void setNext(SkipNode<N> next, int level) {
    this.next[level] = next;
  }

  SkipNode<N> getNext(int level) {
    return this.next[level];
  }

  SkipNode<N> search(N data, int level, boolean print) {
    if (print) {
      System.out.print("Searching for: " + data + " at ");
      print(level);
    }

    SkipNode<N> result = null;
    SkipNode<N> current = this.getNext(level);
    while (current != null && current.data.compareTo(data) < 1) {
      if (current.data.equals(data)) {
        result = current;
        break;
      }

      current = current.getNext(level);
    }

    return result;
  }

  void insert(SkipNode<N> skipNode, int level) {
    SkipNode<N> current = this.getNext(level);
    if (current == null) {
      this.setNext(skipNode, level);
      return;
    }

    if (skipNode.data.compareTo(current.data) < 1) {
      this.setNext(skipNode, level);
      skipNode.setNext(current, level);
      return;
    }

    while (current.getNext(level) != null && current.data.compareTo(skipNode.data) < 1 &&
      current.getNext(level).data.compareTo(skipNode.data) < 1) {

      current = current.getNext(level);
    }

    SkipNode<N> successor = current.getNext(level);
    current.setNext(skipNode, level);
    skipNode.setNext(successor, level);
  }

  void print(int level) {
    System.out.print("level " + level + ": [ ");
    int length = 0;
    SkipNode<N> current = this.getNext(level);
    while (current != null) {
      length++;
      System.out.print(current.data + " ");
      current = current.getNext(level);
    }

    System.out.println("], length: " + length);
  }

}

public class SkipList<T extends Comparable<? super T>> implements SkippableList<T> {

  private final SkipNode<T> head = new SkipNode<>(null);
  private final Random rand = new Random();

  @Override
  public void insert(T data) {
    SkipNode<T> skipNode = new SkipNode<>(data);
    for (int i = 0; i < LEVELS; i++) {
      if (rand.nextInt((int) Math.pow(2, i)) == 0) {
        //insert with prob = 1/(2^i)
        insert(skipNode, i);
      }
    }
  }

  @Override
  public boolean delete(T target) {
    System.out.println("Deleting " + target);
    SkipNode<T> victim = search(target, true);
    if (victim == null) return false;
    victim.data = null;

    for (int i = 0; i < LEVELS; i++) {
      head.refreshAfterDelete(i);
    }

    System.out.println("deleted...");
    return true;
  }

  @Override
  public SkipNode<T> search(T data) {
    return search(data, true);
  }

  @Override
  public void print() {
    for (int i = LEVELS-1; i >= 0 ; i--) {
      head.print(i);
    }
    System.out.println();
  }

  private void insert(SkipNode<T> SkipNode, int level) {
    head.insert(SkipNode, level);
  }

  private SkipNode<T> search(T data, boolean print) {
    SkipNode<T> result = null;
    for (int i = LEVELS-1; i >= 0; i--) {
      if ((result = head.search(data, i, print)) != null) {
        if (print) {
          System.out.println("Found " + data.toString() + " at level " + i + ", so stopped" );
          System.out.println();
        }
        break;
      }
    }

    return result;
  }

  public static void main(String[] args) {
    SkipList<Integer> sl = new SkipList<>();
    int[] data = {4,2,7,0,9,1,3,7,3,4,5,6,0,2,8};
    for (int i : data) {
      sl.insert(i);
    }

    sl.print();
    sl.search(4);

    sl.delete(4);

    System.out.println("Inserting 10");
    sl.insert(10);
    sl.print();
    sl.search(10);
  }

}

Answer 3

Since you've mentioned a List that is both Indexable (I assume you want speedy retrieval) and need to allow duplicates, I would advise you go for a custom Set with a LinkedList or ArrayList perhaps.

You need to have a base Set, an HashSet for example and keep adding values to it. If you face a duplicate, the value of that Set should point to a List. So, that you will have both Speedy retrieval and of course you will store your objects in a psuedo Collection manner.

This should give you good efficiency for retrieval. Ideally if your Keys are not duplicates, you will achieve an O(1) as the retrieval speed.

Answer 4

When you create a ConcurrentSkipListSet, you pass a comparator to the constructor.

new ConcurrentSkipListSet<>(new ExampleComparator());

public class ExampleComparator implements Comparator<Event> {//your impl }

You could create a comparator that will make your SkipListSet behave as a normal List.

Answer 5

You can make use the below to code make your own basic skiplist :

1)Make start and end to represent start and end of skip list.

2)Add the nodes and assign pointers to next based on

if(node is even)
    then ,assign a fast lane pointer with next pointer
  else
    assign only pointer to next node

Java code for basic skip list (you can add more features if you want):

public class MyClass {

    public static void main(String args[]) {
        Skiplist skiplist=new Skiplist();

        Node n1=new Node();
        Node n2=new Node();
        Node n3=new Node();
        Node n4=new Node();
        Node n5=new Node();
        Node n6=new Node();

        n1.setData(1);
        n2.setData(2);
        n3.setData(3);
        n4.setData(4);
        n5.setData(5);
        n6.setData(6);

        skiplist.insert(n1);
        skiplist.insert(n2);
        skiplist.insert(n3);
        skiplist.insert(n4);
        skiplist.insert(n5);
        skiplist.insert(n6);

        /*print all nodes*/
        skiplist.display();
        System.out.println();
        /* print only fast lane node*/
        skiplist.displayFast();
    }
}



class Node{
    private int data;
    private Node one_next;  //contain pointer to next node
    private Node two_next;  //pointer to node after the very next node
    public int getData() {
        return data;
    }
    public void setData(int data) {
        this.data = data;
    }
    public Node getOne_next() {
        return one_next;
    }
    public void setOne_next(Node one_next) {
        this.one_next = one_next;
    }
    public Node getTwo_next() {
        return two_next;
    }
    public void setTwo_next(Node two_next) {
        this.two_next = two_next;
    }


}

class Skiplist{
    Node start;      //start pointer to skip list
    Node head;   
    Node temp_next;  //pointer to store last used fast lane node
    Node end;        //end of skip list
    int length;

    public Skiplist(){
        start=new Node();
        end=new Node();
        length=0;
        temp_next=start;
    }

    public void insert(Node node){
        /*if skip list is empty */
        if(length==0){        
            start.setOne_next(node);
            node.setOne_next(end);
            temp_next.setTwo_next(end);
            head=start;
            length++;
        }
        else{
            length++;
            Node temp=start.getOne_next();
            Node prev=start;

            while(temp != end){
                prev=temp;
                temp=temp.getOne_next();
            }

            /*add a fast lane pointer for even no of nodes*/
            if(length%2==0){
                prev.setOne_next(node);
                node.setOne_next(end);
                temp_next.setTwo_next(node);
                temp_next=node;
                node.setTwo_next(end);
            }
            /*odd no of node will not contain fast lane pointer*/
            else{
                prev.setOne_next(node);
                node.setOne_next(end);
            }


        }
    }

    public void display(){
        System.out.println("--Simple Traversal--");
        Node temp=start.getOne_next();

        while(temp != end){
            System.out.print(temp.getData()+"=>");
            temp=temp.getOne_next();

        }
    }

    public void displayFast(){
        System.out.println("--Fast Lane Traversal--");
        Node temp=start.getTwo_next();
        while(temp !=end){
            System.out.print(temp.getData()+"==>");
            temp=temp.getTwo_next();
        }
    }
}

Output:

--Simple Traversal--

1=>2=>3=>4=>5=>6=>

--Fast Lane Traversal--

2==>4==>6==>

Answer 6

I approve to use TreeList from apache-collections and decorate it with SortedList from Happy Java Libraries https://sourceforge.net/p/happy-guys/wiki/Sorted%20List/

Answer 7

I am not claiming that this is my own implementation. I just cannot remember where I found it. If you know let me know and I will update. This has been working quite well for me:

public class SkipList<T extends Comparable<? super T>> implements Iterable<T> {

Node<T> _head = new Node<>(null, 33);
private final Random rand = new Random();
private int _levels = 1;
private AtomicInteger size = new AtomicInteger(0);

/// <summary>
/// Inserts a value into the skip list.
/// </summary>
public void insert(T value) {
    // Determine the level of the new node. Generate a random number R. The
    // number of
    // 1-bits before we encounter the first 0-bit is the level of the node.
    // Since R is
    // 32-bit, the level can be at most 32.
    int level = 0;
    size.incrementAndGet();
    for (int R = rand.nextInt(); (R & 1) == 1; R >>= 1) {
        level++;
        if (level == _levels) {
            _levels++;
            break;
        }
    }

    // Insert this node into the skip list
    Node<T> newNode = new Node<>(value, level + 1);
    Node<T> cur = _head;
    for (int i = _levels - 1; i >= 0; i--) {
        for (; cur.next[i] != null; cur = cur.next[i]) {
            if (cur.next[i].getValue().compareTo(value) > 0)
                break;
        }

        if (i <= level) {
            newNode.next[i] = cur.next[i];
            cur.next[i] = newNode;
        }
    }
}

/// <summary>
/// Returns whether a particular value already exists in the skip list
/// </summary>
public boolean contains(T value) {
    Node<T> cur = _head;
    for (int i = _levels - 1; i >= 0; i--) {
        for (; cur.next[i] != null; cur = cur.next[i]) {
            if (cur.next[i].getValue().compareTo(value) > 0)
                break;
            if (cur.next[i].getValue().compareTo(value) == 0)
                return true;
        }
    }
    return false;
}

/// <summary>
/// Attempts to remove one occurence of a particular value from the skip
/// list. Returns
/// whether the value was found in the skip list.
/// </summary>
public boolean remove(T value) {

    Node<T> cur = _head;

    boolean found = false;
    for (int i = _levels - 1; i >= 0; i--) {
        for (; cur.next[i] != null; cur = cur.next[i]) {
            if (cur.next[i].getValue().compareTo(value) == 0) {
                found = true;
                cur.next[i] = cur.next[i].next[i];
                break;
            }

            if (cur.next[i].getValue().compareTo(value) > 0)
                break;
        }
    }
    if (found)
        size.decrementAndGet();
    return found;
}

@SuppressWarnings({ "rawtypes", "unchecked" })
@Override
public Iterator<T> iterator() {
    return new SkipListIterator(this, 0);
}

public int size() {
    return size.get();
}

public Double[] toArray() {
    Double[] a = new Double[size.get()];
    int i = 0;
    for (T t : this) {
        a[i] = (Double) t;
        i++;
    }
    return a;
  }

 }

 class Node<N extends Comparable<? super N>> {
public Node<N>[] next;
public N value;

@SuppressWarnings("unchecked")
public Node(N value, int level) {
    this.value = value;
    next = new Node[level];
}

public N getValue() {
    return value;
}

public Node<N>[] getNext() {
    return next;
}

public Node<N> getNext(int level) {
    return next[level];
}

public void setNext(Node<N>[] next) {
    this.next = next;
}
}

class SkipListIterator<E extends Comparable<E>> implements Iterator<E> {
   SkipList<E> list;
   Node<E> current;
   int level;

public SkipListIterator(SkipList<E> list, int level) {
    this.list = list;
    this.current = list._head;
    this.level = level;
}

public boolean hasNext() {
    return current.getNext(level) != null;
}

public E next() {
    current = current.getNext(level);
    return current.getValue();
}

public void remove() throws UnsupportedOperationException {
    throw new UnsupportedOperationException();
}
}