README.md

284. 顶端迭代器

English Version

题目描述

给定一个迭代器类的接口，接口包含两个方法： next() 和 hasNext()。设计并实现一个支持 peek() 操作的顶端迭代器 -- 其本质就是把原本应由 next() 方法返回的元素 peek() 出来。

示例:

假设迭代器被初始化为列表 [1,2,3]。

调用 next() 返回 1，得到列表中的第一个元素。
现在调用 peek() 返回 2，下一个元素。在此之后调用 next() 仍然返回 2。
最后一次调用 next() 返回 3，末尾元素。在此之后调用 hasNext() 应该返回 false。

进阶：你将如何拓展你的设计？使之变得通用化，从而适应所有的类型，而不只是整数型？

解法

定义一个变量 peekElement 专门用来保存下一个值，布尔变量 hasPeeked 标记是否保存了下一个元素。

Python3

# Below is the interface for Iterator, which is already defined for you.
#
# class Iterator:
#     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:
    def __init__(self, iterator):
        """
        Initialize your data structure here.
        :type iterator: Iterator
        """
        self.iterator = iterator
        self.has_peeked = False
        self.peeked_element = None

    def peek(self):
        """
        Returns the next element in the iteration without advancing the iterator.
        :rtype: int
        """
        if not self.has_peeked:
            self.peeked_element = self.iterator.next()
            self.has_peeked = True
        return self.peeked_element

    def next(self):
        """
        :rtype: int
        """
        if not self.has_peeked:
            return self.iterator.next()
        result = self.peeked_element
        self.has_peeked = False
        self.peeked_element = None
        return result

    def hasNext(self):
        """
        :rtype: bool
        """
        return self.has_peeked or self.iterator.hasNext()


# 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].

Java

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html

class PeekingIterator implements Iterator<Integer> {
    private Iterator<Integer> iterator;
    private boolean hasPeeked;
    private Integer peekedElement;

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

    // Returns the next element in the iteration without advancing the iterator.
	public Integer peek() {
        if (!hasPeeked) {
            peekedElement = iterator.next();
            hasPeeked = true;
        }
        return peekedElement;
	}

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

	@Override
	public boolean hasNext() {
	    return hasPeeked || iterator.hasNext();
	}
}

C++

/*
 * 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);
 *
 * 		// 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.
	    hasPeeked = false;
	}

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

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

	bool hasNext() const {
	    return hasPeeked || Iterator::hasNext();
	}
private:
    bool hasPeeked;
    int peekedElement;
};

Go

/*   Below is the interface for Iterator, which is already defined for you.
 *
 *   type Iterator struct {
 *
 *   }
 *
 *   func (this *Iterator) hasNext() bool {
 *		// Returns true if the iteration has more elements.
 *   }
 *
 *   func (this *Iterator) next() int {
 *		// Returns the next element in the iteration.
 *   }
 */

type PeekingIterator struct {
	iter          *Iterator
	hasPeeked     bool
	peekedElement int
}

func Constructor(iter *Iterator) *PeekingIterator {
	return &PeekingIterator{iter, iter.hasNext(), iter.next()}
}

func (this *PeekingIterator) hasNext() bool {
	return this.hasPeeked || this.iter.hasNext()
}

func (this *PeekingIterator) next() int {
	if !this.hasPeeked {
		return this.iter.next()
	}
	this.hasPeeked = false
	return this.peekedElement
}

func (this *PeekingIterator) peek() int {
	if !this.hasPeeked {
		this.peekedElement = this.iter.next()
		this.hasPeeked = true
	}
	return this.peekedElement
}

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