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?

题目大意：

实现一个Integer的Iterator，next() hasNext()和原来的Iterator并没有什么不同，但是peek操作不会改变Iterator指针的位置。

解法：

采用一个next变量进行保存下一次要访问的值，如果是peek操作的话，不改变Iterator指针的位置，而如果是next就会改变。

java:

 

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

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