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

★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★
➤微信公众号：山青咏芝（shanqingyongzhi）
➤博客园地址：山青咏芝（https://www.cnblogs.com/strengthen/）
➤GitHub地址：https://github.com/strengthen/LeetCode
➤原文地址： http://www.javashuo.com/article/p-plgmfxkv-kx.html 
➤若是连接不是山青咏芝的博客园地址，则多是爬取做者的文章。
➤原文已修改更新！强烈建议点击原文地址阅读！支持做者！支持原创！
★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★

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: .

Call  gets you 1, the first element in the list.
Now you call  and it returns 2, the next element. Calling  after that still return 2. 
You call  the final time and it returns 3, the last element. 
Calling  after that should return false.
[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() 操做的顶端迭代器 -- 其本质就是把本来应由 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 。

调用 返回 1，获得列表中的第一个元素。
如今调用  返回 2，下一个元素。在此以后调用 仍然返回 2。
最后一次调用  返回 3，末尾元素。在此以后调用  应该返回 false。
[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，从而适应全部的类型，而不仅是整数型？

---

45ms

 1 class PeekingIterator implements Iterator<Integer> {
 2     Integer next = null;
 3     Iterator<Integer> iterator;
 4     public PeekingIterator(Iterator<Integer> iterator) {
 5         // initialize any member here.
 6         this.iterator = iterator;
 7     }
 8 
 9     // Returns the next element in the iteration without advancing the iterator.
10     public Integer peek() {
11         if(next != null){
12             return next;
13         }
14         
15         this.next = iterator.next();
16         return next;
17     }
18 
19     // hasNext() and next() should behave the same as in the Iterator interface.
20     // Override them if needed.
21     @Override
22     public Integer next() {
23         if(next != null){
24             Integer res = next;
25             next = null;
26             return res;
27         }
28         
29         return iterator.next();
30     }
31 
32     @Override
33     public boolean hasNext() {
34         return next != null || iterator.hasNext();
35     }
36 }

---

46ms

 1 // Java Iterator interface reference:
 2 // https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
 3 class PeekingIterator implements Iterator<Integer> {
 4     
 5     private LinkedList<Integer> list = new LinkedList<Integer>();
 6 
 7     public PeekingIterator(Iterator<Integer> iterator) {
 8         while (iterator.hasNext()) {
 9             list.add(iterator.next());
10         }
11     }
12 
13     // Returns the next element in the iteration without advancing the iterator.
14     public Integer peek() {
15         if (list.isEmpty()) {
16             return null;
17         }
18         return list.getFirst();
19     }
20 
21     // hasNext() and next() should behave the same as in the Iterator interface.
22     // Override them if needed.
23     @Override
24     public Integer next() {
25         return list.removeFirst();
26     }
27 
28     @Override
29     public boolean hasNext() {
30         return !list.isEmpty();
31     }
32 }

---

47ms

 1 // Java Iterator interface reference:
 2 // https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
 3 public class PeekingIterator implements Iterator<Integer> {
 4 
 5     Iterator<Integer> it;
 6     Integer pre;
 7     public PeekingIterator(Iterator<Integer> iterator) {
 8         // initialize any member here.
 9         it=iterator;
10         if(it.hasNext())
11             pre=it.next();
12     }
13 
14     // Returns the next element in the iteration without advancing the iterator.
15     public Integer peek() {
16         return pre;
17         
18     }
19 
20     // hasNext() and next() should behave the same as in the Iterator interface.
21     // Override them if needed.
22     @Override
23     public Integer next() {
24         int res=pre;
25         if(it.hasNext())
26             pre=it.next();
27         else
28             pre=null;
29             
30         return res;
31     }
32 
33     @Override
34     public boolean hasNext() {
35         // return it.hasNext();
36         return pre!=null;
37     }
38 }