Itertors

https://xmruibi.gitbooks.io/interview-preparation-notes/content/DataStructure/Iterator/JumpIterator.html

Implements an iterator in each output it print the number and skip the next one.

public class JumpIterator implements Iterator<Integer> {

    Iterator<Integer> iterator;

    public JumpIterator(Iterator<Integer> iterator) {
        this.iterator = iterator;
    }

    @Override
    public boolean hasNext() {
        return iterator.hasNext();
    }

    @Override
    public Integer next() {
        int res = iterator.next();
        if (iterator.hasNext())
            iterator.next();
        return res;
    }
}

+

https://xmruibi.gitbooks.io/interview-preparation-notes/content/DataStructure/Iterator/EvenIterator.html
Implements an iterator only output the even number.

public class EvenIterator implements Iterator<Integer> {

    Iterator<Integer> iterator;

    public EvenIterator(Iterator<Integer> iterator) {
        this.iterator = iterator;
    }

    @Override
    public boolean hasNext() {
        return iterator.hasNext(); // should put while logic here
    }

    @Override
    public Integer next() {
        int res = 0;
        while (iterator.hasNext() && (res = iterator.next()) % 2 != 0)
            ;
        return res;
    }
}

https://xmruibi.gitbooks.io/interview-preparation-notes/content/DataStructure/Iterator/IteratorFor2DArray.html
Set up a iterator to iterate a 2-dimensional array.
Iterator for 2-D Array

• Iterate the 2-D array by x and y.
• Record current element during hasNext() function.
• Check the row if it is null.

public class DeepIterator{
    int cur; // this is important
    int row = 0, col = 0;

    int[][] listOfLists;

    public DeepIterator(int[][] listOfLists){
        if(listOfLists == null)
            throw new IllegalArgumentException("Null Input");
        this.listOfLists = listOfLists;
    }

    public Integer next(){
        return cur;
    }

    public boolean hasNext(){
        // make sure the row is not null
        while(row < listOfLists.length && col >= listOfLists[row].length) {
                row ++; col = 0;
        }
        if(row < listOfLists.length) {
            cur = listOfLists[row][col++];
            return true;
        }else
            return false;
    }

    public static void main(String[] args) {
        int[][] listOfLists = {
          {},{},{1,2,3},{},{},{2,3,4}
        };
        DeepIterator it = new DeepIterator(listOfLists);
        while(it.hasNext()){
          System.out.println(it.next());
        }
    }
}

https://xmruibi.gitbooks.io/interview-preparation-notes/content/DataStructure/Iterator/IteratorOfIterators.html
Given a Iterator which contain several iterator inside. The task is to iterate all elements inside these iterators

• It's a class implements iterator interface but also contains several iterators.
• Image these iterator is a bucket, the unit is a iterator and the cursor of bucket index is a iteratorelement.
• Set a current index to point at a iterator.

public class Iterators<T> implements Iterator<T>{
    Iterators<T> current;
    Iterators<Iterators<T>> cursor;
    public Iterators(Iterable<Iterator<T>> iterators){
        if(iterators == null)
            throw new IllegalArgumentException("Illegal Argument!");
        this.cursor = iterators;
    }

    @Override
    public T next(){
        return current.next();
    }

    @Override
    public boolean hasNext(){
        if(!current.hasNext())
            current = findNext();
        return current != null && current.hasNext();
    }

    private Iterator findNext(){
        while(cursor.hasNext()){
            current = cursor.next();
            if(current != null && current.hasNext())
                break;
        }
        return current;
    }

    @Override
    public void remove(){
        if(current!=null)
            current.remove();
    }
}

https://xmruibi.gitbooks.io/interview-preparation-notes/content/DataStructure/Iterator/PeekIterator.html

Given an Iterator class interface with methods: next() and hasNext(), design and implement a PeekingIterator that support the peek() operation -- it essentially peek() at the element that will be returned by the next call to next().

+

class PeekingIterator implements Iterator<Integer> {

    int cur;
    Iterator<Integer> it;
    public PeekingIterator(Iterator<Integer> iterator) {
        this.it = iterator;
        cur = it.hasNext() ? it.next() : null; //\\
    }

    // Returns the next element in the iteration without advancing the iterator.
    public Integer peek() {
        return cur;
    }

    // hasNext() and next() should behave the same as in the Iterator interface.
    // Override them if needed.
    @Override
    public Integer next() {
        int res = curl
        cur = it.next() ? it.next() : null;
        return res;
    }

    @Override
    public boolean hasNext() {
        return it.hasNext();
    }
}