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().

Here is an 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.

设计一个有peek操作的iterator，peek操作返回第一个元素，但是不删除。我们有很多种方法，简单的一个借助队列，将iterator中的元素放到队列中，用到队列的peek操作，但这样我们在内存中开辟了一块空间。我们还可以直接利用给定的iterator对象来处理。两种方法的代码如下：
1，借助队列实现

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
class PeekingIterator implements Iterator<Integer> {
    private Queue<Integer> queue;
	public PeekingIterator(Iterator<Integer> iterator) {
	    // initialize any member here.
	    queue = new LinkedList<Integer>();
	    while(iterator.hasNext())
	        queue.offer(iterator.next());
	    
	}

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

	// hasNext() and next() should behave the same as in the Iterator interface.
	// Override them if needed.
	@Override
	public Integer next() {
	    if(!queue.isEmpty())
	        return queue.poll();
	    return Integer.MAX_VALUE;
	}

	@Override
	public boolean hasNext() {
	    return !queue.isEmpty();
	}
}

2，用iterator对象

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

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

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

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