leetcode Peeking Iterator

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?

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题意：

已经给定了Iterator接口

• next()
• hasNext()

让你在此基础上实现PeekingIterator

• peek()：返回下一个元素，但指针不移动到下一个
• next()： 移动到下一个元素x并返回x
• hasNext() ：返回有下一个元素

思路：

分语言而定，详见代码。

C++

定义peek()方法，而peek可以新建一个临时对象，返回其下一个即可。

至于next 和 hasNext直接用父类的方法即可。

// 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.

	}

	// Returns the next element in the iteration without advancing the iterator.
	int peek() {
		return Iterator(*this).next();
	}
};

 

Java

建一个成员变量cur，记录下一个指针指向的值。

• peek： 直接返回cur
• next:  res = cur , 并 判断是否有下一个，若有则赋值为iterator.next，否则null
• hasNext: 判断cur是否为null

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

    Iterator<Integer> iterator;
    Integer cur ;

    public PeekingIterator(Iterator<Integer> iterator) {
        // initialize any member here.
        this.iterator = iterator;
        this.cur = iterator.hasNext()? iterator.next() :null;
    }

    // Returns the next element in the iteration without advancing the iterator.
    public Integer peek() {
        return cur;
    }

    // hasNext() and next() should behave the same as in the Iterator interface.
    // Override them if needed.
    @Override
    public Integer next() {
        Integer res = cur;
        this.cur = iterator.hasNext()? iterator.next() :null;
        return res;
    }

    @Override
    public boolean hasNext() {
        return cur != null;
    }
}

 

Python

同Java

# 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.iterator = iterator
        self.cur = self.iterator.next() if self.iterator.hasNext() else None

    def peek(self):
        """
        Returns the next element in the iteration without advancing the iterator.
        :rtype: int
        """
        return self.cur

    def next(self):
        """
        :rtype: int
        """
        val = self.cur
        self.cur = self.iterator.next() if self.iterator.hasNext() else None
        return val

    def hasNext(self):
        """
        :rtype: bool
        """
        return self.cur is not None
        

# 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].