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?

Problem URL

Solution

給Iterator介面，要求能夠實現返回下一個元素的值，但是指標不前進的peek()方法。

We use a next pointer to denote next element. noSuchElement boolean is used for hasNext(); I also override next() and hasNext();

In constructor, get initial iterator, use private function advanceIter() to assign next value and noSuchElement.

In peek(), simply return next;

In next(), we should move next forward. If iter is over, that is if noSuchElement is true, return exception; else set a temp res = next and call advanceIter();

In hasNext, return !noSuchElement

In advanceIter, if (iter.hasNext) set next to iter.next(), if not, noSuchElement is true.

Code

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
class PeekingIterator implements Iterator<Integer> {
    Integer next;
    Iterator<Integer> iter;
    // noSuchElement is used for determine next element is existed or not.
    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() {
        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;
        }
    }
}

Time Complexity: O(1)
Space Complexity: O(1)

Review