LC.P284[顶端迭代器]

方法一：模拟

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html

class PeekingIterator implements Iterator<Integer> {

    Iterator<Integer> iterator;
    Integer next;

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

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

    @Override
    public Integer next() {
        Integer ans = next;
        next = iterator.hasNext() ? iterator.next() : null;
        return ans;
    }

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

时间复杂度：$O(1)$
空间复杂度：$O(1)$

进阶

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html

class PeekingIterator<E> implements Iterator<E> {

    Iterator<E> iterator;
    E next;

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

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

    @Override
    public E next() {
        E ans = next;
        next = iterator.hasNext() ? iterator.next() : null;
        return ans;
    }

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

时间复杂度：$O(1)$
空间复杂度：$O(1)$