8. Peeking Iterator

// next() will return the value at current itearator and then increments the iterator (cache the next value)
// hasNext() tells is there any element after the current iterator
// peek() will only return the curr element will not increment the iterator



// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html

class PeekingIterator implements Iterator<Integer> {
    
    private Iterator<Integer> itr = null;
    private Integer nextVal = null;
	public PeekingIterator(Iterator<Integer> iterator) {
	    // initialize any member here.
	    itr = iterator;
        if(hasNext()) nextVal = itr.next();
	}
	
    // Returns the next element in the iteration without advancing the iterator.
	public Integer peek() {
        return nextVal;
	}
	
	// hasNext() and next() should behave the same as in the Iterator interface.
	// Override them if needed.
	@Override
	public Integer next() {
        Integer val = nextVal;
        if(itr.hasNext())
            nextVal= itr.next();
        else 
            nextVal = null;
        return val;
	}
	
	@Override
	public boolean hasNext() {
        return nextVal != null || itr.hasNext();
	}
}