284. 窥探迭代器

package LeetCode284;

import java.util.Iterator;

class PeekingIterator implements Iterator {
    private Iterator iterator;
    private Integer nextElement;


    public PeekingIterator(Iterator iterator) {//将迭代器初始化为本类中的迭代器，并将第一个元素赋予nextElement
        // initialize any member here.
       this.iterator=iterator;
       nextElement=iterator.next();


    }

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

    // hasNext() and next() should behave the same as in the Iterator interface.
    // Override them if needed.
    @Override
    public Integer next() {
        Integer ret = nextElement;
        nextElement = iterator.hasNext() ? iterator.next() : null; //利用nextElement=iterator.next()移动指针
        return ret;
    }

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

• 首先需要初始化，并将java中没有实现的方法额外实现
• 内部定义一个变量，来储存当前指针的值，这样peek操作就不会移动指针

进阶问题要求拓展顶端迭代器的设计，使其适用于所有类型，不局限于整数。

对于静态类型语言如 Java、可以通过使用泛型的方式拓展设计，在 PeekingIterator 类中定义泛型，使用时可以用任意类型。

class PeekingIterator implements Iterator {
    private Iterator iterator;
    private E nextElement;

    public PeekingIterator(Iterator iterator) {
        this.iterator = iterator;
        nextElement = iterator.next();
    }
    
    public E peek() {
        return nextElement;
    }
    
    @Override
    public E next() {
        E ret = nextElement;
        nextElement = iterator.hasNext() ? iterator.next() : null;
        return ret;
    }
    
    @Override
    public boolean hasNext() {
        return nextElement != null;
    }
}