284. 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.

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

Solution:

Use a variable to track the peek element which is the next element. So if peekElement is not null, means it has been peeked, so we just return the peekElement, if it is null, we assign peekElement=iterator.next(). The problem now is when we call next(), we need to check peekElement first, if it is null, means the next element is cached in the peekElement, we return peekElement and set peekElement to null. To check if the iterator hasNext(), we have to check if there is a cached peekElement and if the iterator hasNext().

 // Java Iterator interface reference:  
 // https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html  
 class PeekingIterator implements Iterator<Integer> {  
     Integer peekElement;  
     Iterator<Integer> myIterator;  
      public PeekingIterator(Iterator<Integer> iterator) {  
        // initialize any member here.  
        peekElement=null;  
     myIterator=iterator;  
      }  
   // Returns the next element in the iteration without advancing the iterator.  
      public Integer peek() {  
     if(peekElement==null) peekElement=myIterator.next();  
     return peekElement;  
      }  
      // hasNext() and next() should behave the same as in the Iterator interface.  
      // Override them if needed.  
      @Override  
      public Integer next() {  
        if(peekElement!=null){  
       Integer temp=peekElement;  
       peekElement=null;  
       return temp;  
     }  
     else return myIterator.next();  
      }  
      @Override  
      public boolean hasNext() {  
        return myIterator.hasNext() || peekElement!=null;  
      }  
 }