284. Peeking Iterator

本来的Iterator类里面是自带next()和hasNext()的，但是现在加了peek()之后会调用一次iterator.next(),然后iterator的顺序就乱了。

所以一旦被peek过我们就需要把这个被next()弹出的元素记录下来，如果再调用next就返回这个临时的元素，再调用peek就还返回这个元素，再调用hasNext就返回有没有被peek过或者hasNext()本身

class PeekingIterator implements Iterator<Integer> {
    private Iterator<Integer> it;
    private Integer next;
    private boolean isPeeked;

    public PeekingIterator(Iterator<Integer> iterator) {
        // initialize any member here.
        it = iterator;
        isPeeked = false;
    }

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

    @Override
    public boolean hasNext() {
        return isPeeked? true: it.hasNext();
    }
}

bug记录：

peek()本身的时候也要检查有没有被peek过，否则会一直往后面看去，就会丢失元素