Leetcode 284. Peeking Iterator-优快云博客

本文介绍如何设计并实现一个支持peek()操作的迭代器，该操作可以在调用next()方法之前预览即将返回的元素。通过具体示例和Java代码实现，展示了如何在迭代器中加入预览功能，同时保持hasNext()和next()方法的正常运行。

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题目描述

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().

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.
Follow up: How would you extend your design to be generic and work with all types, not just integer?

代码

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

    private Iterator<Integer> iterator;
    private Integer next;
    private boolean empty;
    public PeekingIterator(Iterator<Integer> iterator) {
        // initialize any member here.
        this.iterator = iterator;
        if (iterator.hasNext()) {
            empty = false;
            next = iterator.next();
        } else {
            empty = true;
            next = null;
        }
    }

    // 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 tmp = next;
        if (iterator.hasNext()) {
            next = iterator.next();
        } else {
            next = null;
            empty = true;
        }
        return tmp;
    }

    @Override
    public boolean hasNext() {
        return !empty;
    }
}