LeetCode Peeking Iterator

原題連接在這裏：https://leetcode.com/problems/peeking-iterator/

題目：

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().

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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.

題解：

設計一個是否被peek過得flag, isPeeked with initialization as false.

同時保存peek過得值peekVal. 若果isPeeked == true 已經peek過，next()直接返回peekVal, hasNext()直接看isPeek爲true 就返回 true.

Time Complexity: peek, O(1). next, O(1). hasNext, O(1).

Space: O(1). 

AC Java:

 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     Iterator<Integer> it;
 5     boolean isPeeked;
 6     int peekVal;
 7     
 8     public PeekingIterator(Iterator<Integer> iterator) {
 9         // initialize any member here.
10         this.it = iterator;
11         isPeeked = false;
12     }
13 
14     // Returns the next element in the iteration without advancing the iterator.
15     public Integer peek() {
16         if(!isPeeked){
17             isPeeked = true;
18             if(it.hasNext()){
19                 peekVal = it.next();
20             }else{
21                 throw new IllegalStateException("iterator doesn't has next value.");
22             }
23         }
24         return peekVal;
25     }
26 
27     // hasNext() and next() should behave the same as in the Iterator interface.
28     // Override them if needed.
29     @Override
30     public Integer next() {
31         if(isPeeked){
32             isPeeked = false;
33             return peekVal;
34         }
35         return it.next();
36     }
37 
38     @Override
39     public boolean hasNext() {
40         return isPeeked || it.hasNext();
41     }
42 }