[Java]LeetCode284. 顶端迭代器 | Peeking Iterator

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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?

---

给定一个迭代器类的接口，接口包含两个方法： next() 和 hasNext()。设计并实现一个支持 peek() 操作的顶端迭代器 -- 其本质就是把原本应由 next() 方法返回的元素 peek() 出来。

示例:

假设迭代器被初始化为列表 [1,2,3]。

调用 next() 返回 1，得到列表中的第一个元素。
现在调用 peek() 返回 2，下一个元素。在此之后调用 next() 仍然返回 2。
最后一次调用 next() 返回 3，末尾元素。在此之后调用 hasNext() 应该返回 false。

进阶：你将如何拓展你的设计？使之变得通用化，从而适应所有的类型，而不只是整数型？

---

45ms

class PeekingIterator implements Iterator<Integer> {
    Integer next = null;
    Iterator<Integer> iterator;
    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(next != null){
            return next;
        }
        
        this.next = iterator.next();
        return next;
    }

    // hasNext() and next() should behave the same as in the Iterator interface.
    // Override them if needed.
    @Override
    public Integer next() {
        if(next != null){
            Integer res = next;
            next = null;
            return res;
        }
        
        return iterator.next();
    }

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

---

46ms

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
class PeekingIterator implements Iterator<Integer> {
    
    private LinkedList<Integer> list = new LinkedList<Integer>();

    public PeekingIterator(Iterator<Integer> iterator) {
        while (iterator.hasNext()) {
            list.add(iterator.next());
        }
    }

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

    // hasNext() and next() should behave the same as in the Iterator interface.
    // Override them if needed.
    @Override
    public Integer next() {
        return list.removeFirst();
    }

    @Override
    public boolean hasNext() {
        return !list.isEmpty();
    }
}

---

47ms

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

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

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

    // hasNext() and next() should behave the same as in the Iterator interface.
    // Override them if needed.
    @Override
    public Integer next() {
        int res=pre;
        if(it.hasNext())
            pre=it.next();
        else
            pre=null;
            
        return res;
    }

    @Override
    public boolean hasNext() {
        // return it.hasNext();
        return pre!=null;
    }
}