问题描述：
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().

Here is an 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.

分析：对于迭代器而言，没有peek的函数项，同样也没有添加的功能。所以能够获取数据的唯一渠道就是通过next，但是一旦next之后，迭代器中的元素就会删掉一个，那如何保持被删掉的元素存活呢？就是在属性中添加个值，来保持alive，同时使用isValid来验证该值是否还有效。

代码如下：288ms

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

Iterator<Integer> iterator;
    int value;
    boolean isValid;
    public PeekingIterator(Iterator<Integer> iterator) {
        // initialize any member here.
        this.iterator = iterator;
        value = -1;
        isValid = false;
    }

    // Returns the next element in the iteration without advancing the iterator.
    public Integer peek() {
        if(isValid)
            return value;
        int val = iterator.next();
        value = val;
        isValid = true;
        return val;
    }

    // hasNext() and next() should behave the same as in the Iterator interface.
    // Override them if needed.
    @Override
    public Integer next() {
        if(isValid){
            isValid = false;
            return value;
        }
        return iterator.next();
    }

    @Override
    public boolean hasNext() {
        if(isValid)
            return true;
        return iterator.hasNext();
    }
}