284. Peeking Iterator

Posted 2020-07-03 warmland

本来的Iterator类里面是自带next()和hasNext()的，但是现在加了peek()之后会调用一次iterator.next(),然后iterator的顺序就乱了。

所以一旦被peek过我们就需要把这个被next()弹出的元素记录下来，如果再调用next就返回这个临时的元素，再调用peek就还返回这个元素，再调用hasNext就返回有没有被peek过或者hasNext()本身

 1 class PeekingIterator implements Iterator<Integer> {
 2     private Iterator<Integer> it;
 3     private Integer next;
 4     private boolean isPeeked;
 5 
 6     public PeekingIterator(Iterator<Integer> iterator) {
 7         // initialize any member here.
 8         it = iterator;
 9         isPeeked = false;
10     }
11 
12     // Returns the next element in the iteration without advancing the iterator.
13     public Integer peek() {
14         if (isPeeked) {
15             return next;
16         } else {
17             isPeeked = true;
18             next = it.next();
19             return next;
20         }
21     }
22 
23     // hasNext() and next() should behave the same as in the Iterator interface.
24     // Override them if needed.
25     @Override
26     public Integer next() {
27         Integer res;
28         if(isPeeked) {
29             res = next;
30         } else {
31             res = it.next();
32         }
33         isPeeked = false;
34         return res;
35     }
36 
37     @Override
38     public boolean hasNext() {
39         return isPeeked? true: it.hasNext();
40     }
41 }

bug记录：

peek()本身的时候也要检查有没有被peek过，否则会一直往后面看去，就会丢失元素