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().
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.
一般的Iterator只有两个方法next和hasNext。现在要实现一个遍历器可以只看看下一个元素是什么但是不走这一步。
最简单的方法是把传进来的数据先用遍历器遍历一遍，存到一个ArrayList里。。。。这样我们可以随心所欲的取值

class PeekingIterator implements Iterator<Integer> {

    int index = -1;
    ArrayList<Integer> data = new ArrayList<Integer>();
    public PeekingIterator(Iterator<Integer> iterator) {
        // initialize any member here.
        while (iterator.hasNext())
            data.add(iterator.next());
    }

    // Returns the next element in the iteration without advancing the iterator.
    public Integer peek() {
        System.out.println("peek"+index);
        if (hasNext())
            return data.get(index+1);
        return null;
    }

    // hasNext() and next() should behave the same as in the Iterator interface.
    // Override them if needed.
    @Override
    public Integer next() {
        System.out.println("next"+index);
        index += 1;
        return data.get(index);
    }

    @Override
    public boolean hasNext() {
        System.out.println("hasNext"+index);
        if (index+1<data.size())
            return true;
        return false;
    }
}

但是人家明显不是这个意思。。。
我们可以使用一个变量保存住下一个要遍历的，每次peak就返回这个变量，每次next就返回这个变量并往后遍历一个赋给这个变量

class PeekingIterator implements Iterator<Integer> {
    private Integer next = null;
    private Iterator<Integer> iter;

    public PeekingIterator(Iterator<Integer> iterator) {
        // initialize any member here.
        iter = iterator;
        if (iter.hasNext())
            next = iter.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 = iter.hasNext() ? iter.next() : null;
        return res; 
    }

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