Leetcode284-peekingIterator

Description

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().

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

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
class PeekingIterator implements Iterator<Integer> {

    private Iterator<Integer> it;
    private Integer cur;
	public PeekingIterator(Iterator<Integer> iterator) {
	    // initialize any member here.
	    it = iterator;
        if (it.hasNext()) cur = it.next();
        else cur = null;
	}

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

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

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

Method 2, more better

import java.util.NoSuchElementException;
class PeekingIterator implements Iterator<Integer> {
    Integer next;
    Iterator<Integer> iter;
    boolean noSuchElement;

    public PeekingIterator(Iterator<Integer> iterator) {
	// initialize any member here.
	iter = iterator;
        advanceIter();
    }

    // Returns the next element in the iteration without advancing the iterator.
    public Integer peek() {
        // you should confirm with interviewer what to return/throw
        // if there are no more values
        return next;
    }

    // hasNext() and next() should behave the same as in the Iterator interface.
    // Override them if needed.
    @Override
    public Integer next() {
        if (noSuchElement)
            throw new NoSuchElementException();
        Integer res = next;
        advanceIter();
        return res;
    }

    @Override 
    public boolean hasNext() {
        return !noSuchElement;
    }
    
    private void advanceIter() {
        if (iter.hasNext()) {
            next = iter.next();
        } else {
            noSuchElement = true;
        }
    }
}

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