[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:

[1,2,3]
next()
peek()
next()
next()
hasNext()

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:

[1,2,3]
next() 
peek()
next() 
next()
hasNext()

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;
    }
}