284. Peeking Iterator

题目：顶端迭代器

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.

Hint:

1. Think of "looking ahead". You want to cache the next element.
2. Is one variable sufficient? Why or why not?
3. Test your design with call order of peek() before next() vs next() before peek().
4. For a clean implementation, check out Google's guava library source code.

Follow up: How would you extend your design to be generic and work with all types, not just integer?

题意：

给定一个包含由next()和hasNext()接口的迭代器类，设计和实现一个支持peek()操作的顶端迭代器，调用peek()操作的元素将会在下一次调用next()接口的时候被返回。

这儿有一个例子，假设这个迭代器被初始化为[1,2,3]这个列表的开始位置。

调用next()接口会返回列表的第一个元素1给你。

现在你调用peek()接口将会返回元素2给你，调用next()接口之后还是返回2给你。

最后一次调用next()接口将会返回列表的最后一个元素3给你。调用hasNext()接口之后应该返回false给你。

Hint：

1、可以认为是“查看开始的元素”。你想要缓存的下一个元素；

2、一个变量是否足够？为什么行或者为什么不行？

3、测试对比你的设计先调用peek()接口再调用next()接口和先调用next()接口再调用peek()接口有什么不同；

4、一个简单的实现，查看Google's guava library source code.

Follow up：

你将如何扩展你的设计达到通用的效果，不仅仅是整数，而是其他所有类型都适用。

思路一：

这道题让我们实现一个顶端迭代器，在普通的迭代器类Iterator的基础上增加了peek的功能，就是返回查看下一个值的功能，但是不移动指针，next()函数才会移动指针，那我们可以定义一个变量专门来保存下一个值，再用一个bool型变量标记是否保存了下一个值，再调用原来的一些成员函数，就可以实现这个顶端迭代器了。

代码：C++版：4ms

// Below is the interface for Iterator, which is already defined for you.
// **DO NOT** modify the interface for Iterator.
class Iterator {
    struct Data;
	Data* data;
public:
	Iterator(const vector<int>& nums);
	Iterator(const Iterator& iter);
	virtual ~Iterator();
	// Returns the next element in the iteration.
	int next();
	// Returns true if the iteration has more elements.
	bool hasNext() const;
};


class PeekingIterator : public Iterator {
public:
	PeekingIterator(const vector<int>& nums) : Iterator(nums) {
	    // Initialize any member here.
	    // **DO NOT** save a copy of nums and manipulate it directly.
	    // You should only use the Iterator interface methods.
	    _flag = false;
	}

    // Returns the next element in the iteration without advancing the iterator.
	int peek() {
        if (!_flag) {
            _value = Iterator::next();
            _flag = true;
        }
        return _value;
	}

	// hasNext() and next() should behave the same as in the Iterator interface.
	// Override them if needed.
	int next() {
	    if (!_flag) return Iterator::next();
	    _flag = false;
	    return _value;
	}

	bool hasNext() const {
	    if (_flag) return true;
	    if (Iterator::hasNext()) return true;
	    return false;
	}
private:
    int _value;
    int _flag;
};

思路二：

以上方法使用了两个变量_flag和_value，以下这没有使用变量，直接调用原数据结构的变量值完成。

代码：C++版：4ms

// Below is the interface for Iterator, which is already defined for you.
// **DO NOT** modify the interface for Iterator.
class Iterator {
    struct Data;
	Data* data;
public:
	Iterator(const vector<int>& nums);
	Iterator(const Iterator& iter);
	virtual ~Iterator();
	// Returns the next element in the iteration.
	int next();
	// Returns true if the iteration has more elements.
	bool hasNext() const;
};


class PeekingIterator : public Iterator {
public:
	PeekingIterator(const vector<int>& nums) : Iterator(nums) {
	    // Initialize any member here.
	    // **DO NOT** save a copy of nums and manipulate it directly.
	    // You should only use the Iterator interface methods.
	    _flag = false;
	}

    // Returns the next element in the iteration without advancing the iterator.
	int peek() {
        if (!_flag) {
            _value = Iterator::next();
            _flag = true;
        }
        return _value;
	}

	// hasNext() and next() should behave the same as in the Iterator interface.
	// Override them if needed.
	int next() {
	    if (!_flag) return Iterator::next();
	    _flag = false;
	    return _value;
	}

	bool hasNext() const {
	    if (_flag) return true;
	    if (Iterator::hasNext()) return true;
	    return false;
	}
private:
    int _value;
    int _flag;
};