LeetCode 题解(213) : Peeking Iterator

题目：

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?

题解：

用已有的Iterator类实现PeekIterator类，需要提前移动iterator指向下一个元素，用额外变量nextValue记录用于peek返回的值，用一个boolean变量判断是否到尾部。

C++版：

// 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.
	    iter = new Iterator(nums);
	    if(iter->hasNext()) {
	        nextValue = iter->next();
	    } else {
	        end = true;
	    }
	}

    // Returns the next element in the iteration without advancing the iterator.
	int peek() {
        if(end == false) {
            return nextValue;
        } else {
            return 0;
        }
	}

	// hasNext() and next() should behave the same as in the Iterator interface.
	// Override them if needed.
	int next() {
	    int current = nextValue;
	    if(iter->hasNext()) {
	        nextValue = iter->next();
	    } else {
	        end = true;
	    }
	    return current;
	}

	bool hasNext() const {
	    if(end == true)
	        return false;
	    else
	        return true;
	}
private:
    Iterator* iter;
    int nextValue = 0;
    bool end = false;
};

Java版：

import java.util.NoSuchElementException;
// 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 = null;
    private int nextValue = 0;
    private boolean end = false;
	public PeekingIterator(Iterator<Integer> iterator) {
	    // initialize any member here.
	    iter = iterator;
	    if(iter.hasNext()) {
	        nextValue = iter.next();
	    } else {
	        end = true;
	    }
	}

    // Returns the next element in the iteration without advancing the iterator.
	public Integer peek() {
        if(end == false)
            return nextValue;
        else
            throw new NoSuchElementException();
	}

	// hasNext() and next() should behave the same as in the Iterator interface.
	// Override them if needed.
	@Override
	public Integer next() {
	    int current = nextValue;
	    if(iter.hasNext()) {
	        nextValue = iter.next();
	    } else {
	        end = true;
	    }
	    return current;
	}

	@Override
	public boolean hasNext() {
	    if(end == true)
	        return false;
	   else
	        return true;
	}
}

Python版：

# Below is the interface for Iterator, which is already defined for you.
#
# class Iterator(object):
#     def __init__(self, nums):
#         """
#         Initializes an iterator object to the beginning of a list.
#         :type nums: List[int]
#         """
#
#     def hasNext(self):
#         """
#         Returns true if the iteration has more elements.
#         :rtype: bool
#         """
#
#     def next(self):
#         """
#         Returns the next element in the iteration.
#         :rtype: int
#         """

class PeekingIterator(object):
    def __init__(self, iterator):
        """
        Initialize your data structure here.
        :type iterator: Iterator
        """
        self.iter = iterator
        self.nextValue = 0
        self.end = False
        if self.iter.hasNext():
            self.nextValue = self.iter.next()
        else:
            self.end = True

    def peek(self):
        """
        Returns the next element in the iteration without advancing the iterator.
        :rtype: int
        """
        if not self.end:
            return self.nextValue
        else:
            return 0

    def next(self):
        """
        :rtype: int
        """
        current = self.nextValue
        if self.iter.hasNext():
            self.nextValue = self.iter.next()
        else:
            self.end = True
        return current

    def hasNext(self):
        """
        :rtype: bool
        """
        if self.end:
            return False
        return True

# Your PeekingIterator object will be instantiated and called as such:
# iter = PeekingIterator(Iterator(nums))
# while iter.hasNext():
#     val = iter.peek()   # Get the next element but not advance the iterator.
#     iter.next()         # Should return the same value as [val].