Peeking Iterator

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().

---

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.

Show Hint 

Follow up: How would you extend your design to be generic and work with all types, not just integer?

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
class PeekingIterator implements Iterator<Integer> {
    LinkedList<Integer> queue = new LinkedList<Integer>();
 
public PeekingIterator(Iterator<Integer> iterator) {
   // initialize any member here.
   while (iterator != null && iterator.hasNext()) {
       queue.offer(iterator.next());
   }  
}

    // Returns the next element in the iteration without advancing the iterator.
public Integer peek() {
        return queue.peek();
}

// hasNext() and next() should behave the same as in the Iterator interface.
// Override them if needed.
@Override
public Integer next() {
   return queue.poll();
}

@Override
public boolean hasNext() {
   return !queue.isEmpty();
}
}

=======

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
class PeekingIterator implements Iterator<Integer> {
    Iterator<Integer> iterator;
    Integer peek;
   
public PeekingIterator(Iterator<Integer> iterator) {
   this.iterator = iterator;
   this.peek = null;
}

    // Returns the next element in the iteration without advancing the iterator.
public Integer peek() {
        if (peek == null) {
            peek = iterator.next();
        }
        return peek;
}

// hasNext() and next() should behave the same as in the Iterator interface.
// Override them if needed.
@Override
public Integer next() {
   if (peek != null) {
       Integer res = peek;
       peek = null;
       return res;
   } else {
       return iterator.next();
   }
}

@Override
public boolean hasNext() {
   return peek != null || iterator.hasNext();
}
}