Leetcode 284 - Google Interview - Peek Iterator

https://leetcode.com/problems/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?

https://discuss.leetcode.com/topic/24883/concise-java-solution

I used the exactly same solution. But I do feel using null to check if we have reached the end of iterator is not 100% right. In theory, null could be a valid element.
An slightly alternative approach to use boolean indicating end of iterator:

private Integer next;
private Iterator<Integer> itr;
private boolean done = false;

public PeekingIterator(Iterator<Integer> iterator) {
    itr = iterator;
    if (iterator.hasNext())
        next = itr.next();
    else
        done = true;
}

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

// hasNext() and next() should behave the same as in the Iterator interface.
// Override them if needed.
@Override
public Integer next() {
    Integer result = next;
    if (itr.hasNext()) {
        next = itr.next();
    } else {
        next = null;
        done = true;
    }
    return result;
}

@Override
public boolean hasNext() {
    return !done;
}

class PeekingIterator implements Iterator<Integer> {  
    private Integer next = null;
    private Iterator<Integer> iter;

    public PeekingIterator(Iterator<Integer> iterator) {
        // initialize any member here.
        iter = iterator;
        if (iter.hasNext())
            next = iter.next();
    }
    
    // Returns the next element in the iteration without advancing the iterator. 
    public Integer peek() {
        return next; 
    }

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

    @Override
    public boolean hasNext() {
        return next != null;
    }
}

cache the next element. If next is null, there is no more elements in iterator.

X.

class PeekingIterator implements Iterator<Integer> {

    Queue<Integer> iter = new LinkedList<>(); 
    
 public PeekingIterator(Iterator<Integer> iterator) {
     while (iterator.hasNext()) 
         iter.offer(iterator.next());   
 }

 public Integer peek() {
            return iter.peek(); 
 }

 @Override
 public Integer next() {
     return iter.poll(); 
 }

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

Google Interview - Peek Iterator - 我的博客 - ITeye技术网站

Suppose you have a Iterator class with has_next() and get_next() methods.

Please design and implement a PeekIterator class as a wrapper of Iterator and provide a peek() method.

When calling peek(), the user will only get the current element without moving forward the iterator.

1. public class PeekIterator {  
2.     Iterator it;  
3.     Integer top;  
4.     public PeekIterator(Iterator it) {  
5.         this.it = it;  
6.     }  
7.       
8.     public int peek() {  
9.         if(top == null)  
10.             top = (Integer)it.next();  
11.         return top;  
12.     }  
13.       
14.     public boolean has_next() {  
15.         return top != null ? true : it.hasNext();  
16.     }  
17.       
18.     public int get_next() {  
19.         Integer val = top;  
20.         if(val == null)   
21.             val = (Integer)it.next();  
22.         top = null;  
23.         return val;  
24.     }  
25. }  

http://www.cnblogs.com/grandyang/p/4825068.html
http://likemyblogger.blogspot.com/2015/09/leetcode-284-peeking-iterator.html
Read full article from Google Interview - Peek Iterator - 我的博客 - ITeye技术网站