284. Peeking Iterator

Question

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().

Example:

Assume that the iterator is initialized to the beginning of the list: [1,2,3].

Call next() gets you 1, the first element in the list.
Now you call peek() and it returns 2, the next element. Calling next() after that still return 2.
You call next() the final time and it returns 3, the last element.
Calling hasNext() after that should return false.

Thinking:

• Method 1:

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
class PeekingIterator implements Iterator<Integer> {
    private Iterator<Integer> it;
    private boolean peeked = false;
    private Integer next = null;
	public PeekingIterator(Iterator<Integer> iterator) {
	    // initialize any member here.
	    this.it = iterator;
	}

    // Returns the next element in the iteration without advancing the iterator.
	public Integer peek() {
        if(peeked)
            return next;
        else{
            peeked = true;
            next = it.next();
            return next;
        }
	}

	// hasNext() and next() should behave the same as in the Iterator interface.
	// Override them if needed.
	@Override
	public Integer next() {
	    if(peeked){
            peeked = false;
            Integer res = next;
            next = null;
            return res;
        }else{
            return it.next();
        }
	}

	@Override
	public boolean hasNext() {
	    if(peeked){
            return true;
        }else
            return it.hasNext();
	}
}

Second time

1. I used a queue to save the values that can be gotten from the iterator.
2. peek means getting the first value in the queue.
3. next means poll the queue.

   // Java Iterator interface reference:
   // https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
   class PeekingIterator implements Iterator<Integer> {
    Iterator<Integer> it = null;
    LinkedList<Integer> record = null;
    public PeekingIterator(Iterator<Integer> iterator) {
        // initialize any member here.
        this.it = iterator;
        record = new LinkedList<>();
        while(it.hasNext()){
            int num = it.next();
            this.record.add(num);
        }
    }
    // Returns the next element in the iteration without advancing the iterator.
    public Integer peek() {
        return record.get(0);
    }
    // hasNext() and next() should behave the same as in the Iterator interface.
    // Override them if needed.
    @Override
    public Integer next() {
        return record.poll();
    }
    @Override
    public boolean hasNext() {
        return this.record.size() > 0;
    }
   }