LeetCode/problems/java/peeking-iterator.java

class PeekingIterator implements Iterator<Integer> {
    private Iterator<Integer> it;

    private Integer pVal;
    private boolean pHasNext;

    public PeekingIterator(Iterator<Integer> iterator) {
        it = iterator;
        next();
    }

    public Integer peek() {
        return pVal;
    }

    @Override
    public Integer next() {
        pHasNext = it.hasNext();

        Integer oldValue = pVal;
        if (pHasNext) {
            pVal = it.next();
        }

        return oldValue;
    }

    @Override
    public boolean hasNext() {
        return pHasNext;
    }
}