284. Peeking Iterator

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().

Hint:

1. Think of "looking ahead". You want to cache the next element.
2. Is one variable sufficient? Why or why not?
3. Test your design with call order of peek() before next() vs next() before peek().
4. For a clean implementation, check out Google's guava library source code.

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
class PeekingIterator implements Iterator<Integer> {
    private Iterator<Integer> iterator;
    private Integer next = null;

	public PeekingIterator(Iterator<Integer> iterator) {
	    // initialize any member here.
	    this.iterator = iterator;
	    if(iterator.hasNext()) {
	        this.next = iterator.next();
	    }
	}

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

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

	@Override
	public boolean hasNext() {
	    return (next != null);
	}
}