Peeking Iterator 解答

Question

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.

Solution

Key to the solution is to cache status and value ahead.

 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     private Iterator<Integer> iterator;
 5     private int current;
 6     private int peekValue;
 7     private boolean hasNext;
 8 
 9     public PeekingIterator(Iterator<Integer> iterator) {
10         // initialize any member here.
11         this.iterator = iterator;
12         hasNext = this.iterator.hasNext();
13         if (hasNext)
14             peekValue = this.iterator.next();
15     }
16 
17     // Returns the next element in the iteration without advancing the iterator.
18     public Integer peek() {
19         return peekValue;
20     }
21 
22     // hasNext() and next() should behave the same as in the Iterator interface.
23     // Override them if needed.
24     @Override
25     public Integer next() {
26         current = peekValue;
27         hasNext = iterator.hasNext();
28         // Note here we need check hasNext, otherwise it will report runtime error
29         if (hasNext)
30             peekValue = iterator.next();
31         return current;
32     }
33 
34     @Override
35     public boolean hasNext() {
36         return hasNext;
37     }
38 }