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

    private Iterator<Integer> it;
    private Integer peekedValue;

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

    // Returns the next element in the iteration without advancing the iterator.
    // 缓存下一次的值并返回
    public Integer peek() {
        if (this.peekedValue == null && it.hasNext()){
            this.peekedValue = it.next();
        }
        return this.peekedValue;
    }

    // hasNext() and next() should behave the same as in the Iterator interface.
    // Override them if needed.
    // 如果peeked存在,说明原迭代器已经走了一步,直接返回peeked值并置空, 保证操作幂等
    @Override
    public Integer next() {
        if (this.peekedValue != null){
            Integer res = this.peekedValue;
            this.peekedValue = null;
            return res;
        }
        return this.it.next();
    }


    // 如果存在peeked缓存则说明至少还有一个
    // 不存在peeked缓存时与原it一致
    @Override
    public boolean hasNext() {
        return this.peekedValue != null || this.it.hasNext();
    }
}