实现迭代器 Peeking Iterator

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问题：

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().

Here is an 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?

解决：

① 给定一个迭代器的接口，接口支持两个方法，next(), hasNext(),next()返回下一个元素，指针后移一位，hasNext返回是否还有下一个元素，要求在这个接口上做一层封装，使得支持peek()方法，即获取下一个元素但是不移动指针。

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
public class PeekingIterator implements Iterator<Integer> { //102ms
    Iterator<Integer> iterator;
    boolean peeked = false;//标记是否有缓存
    Integer top;//缓存next（）方法的返回值
    public PeekingIterator(Iterator<Integer> iterator) {
        // initialize any member here.
        this.iterator = iterator;
    }
    // Returns the next element in the iteration without advancing the iterator.
    public Integer peek() {
        if (! peeked){
            top = iterator.next();
            peeked = true;
        }
        return top;
    }
    // hasNext() and next() should behave the same as in the Iterator interface.
    // Override them if needed.
    @Override
    public Integer next() {
        if (! peeked){
            return iterator.next();
        }else{
            peeked = false;
            return top;
        }
    }
    @Override
    public boolean hasNext() {
        if (peeked){
            return true;
        }else {
            return iterator.hasNext();
        }
    }
}

② 

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
public class PeekingIterator implements Iterator<Integer> { //97ms
    Iterator<Integer> iterator;
    Integer next;
    public PeekingIterator(Iterator<Integer> iterator) {
        // initialize any member here.
        this.iterator = iterator;
        if (iterator.hasNext()){
            next = (Integer)iterator.next();
        }
    }
    // 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 res = next;
        next = iterator.hasNext() ? (Integer) iterator.next() : null;
        return res;
    }
    @Override
    public boolean hasNext() {
        return next != null;
    }
}

转载于:https://my.oschina.net/liyurong/blog/1591969