284. Peeking Iterator

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

Iteration

添加一个nextVal变量，指向next即可。注意判断hasNext的时候有两种情况！

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

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

    // Returns the next element in the iteration without advancing the iterator.
    public Integer peek() {
        if (nextVal == null && hasNext()) {
            nextVal = next();
        }
        
        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;
        nextVal = null;
        return val != null ? val : iterator.next();
    }

    @Override
    public boolean hasNext() {
        return nextVal != null || iterator.hasNext();
    }
}

也可以让nextVal始终指向下一个元素，这样实现起来方便一点，但需要注意调用iterator的方法，而非PeekingIterator的方法。

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

    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() {
        Integer res = next;
        advanceIter();
        return res;
    }

    @Override
    public boolean hasNext() {
        return next != null;
    }
    
    private void advanceIter() {
        if (iter.hasNext()) {   // use iter.# instead of #
            next = iter.next();
        } else {
            next = null;
        }
    }
}

考虑list中有null值的终极写法：

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

class PeekingIterator implements Iterator<Integer> {
    private Iterator<Integer> iter;
    private Integer next;    // always valid
    private 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 {
            next = null;
            noSuchElement = true;
        }
    }
}