[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.


Think of "looking ahead". You want to cache the next element.
Is one variable sufficient? Why or why not?
Test your design with call order of peek() before next() vs next() before peek().
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?


给定一个迭代器类接口,包含方法: next()hasNext(),设计并实现一个PeekingIterator支持peek()操作 -- 其本质就是把原本应由next()方法返回的元素peek()出来。

这里有一个例子。假设迭代器初始化为list:[1, 2, 3]。








本题目考察设计模式中的装饰器模式(Decorator Pattern)

参阅维基百科Decorator Pattern词条:https://en.wikipedia.org/wiki/Decorator_pattern

In object-oriented programming, the decorator pattern (also known as Wrapper, an alternative naming shared with the Adapter pattern) is a design pattern that allows behavior to be added to an individual object, either statically or dynamically, without affecting the behavior of other objects from the same class. The decorator pattern is often useful for adhering to the Single Responsibility Principle, as it allows functionality to be divided between classes with unique areas of concern.







# 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.peekFlag = False
        self.nextElement = None

    def peek(self):
        Returns the next element in the iteration without advancing the iterator.
        :rtype: int
        if not self.peekFlag:
            self.nextElement = self.iter.next()
            self.peekFlag = True
        return self.nextElement

    def next(self):
        :rtype: int
        if not self.peekFlag:
            return self.iter.next()
        nextElement = self.nextElement
        self.peekFlag = False
        self.nextElement = None
        return nextElement

    def hasNext(self):
        :rtype: bool
        return self.peekFlag or self.iter.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 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 peekFlag = false;
    private Integer nextElement = null;
    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 (!peekFlag) {
            nextElement = iterator.next();
            peekFlag = true;
        return nextElement;

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

    public boolean hasNext() {
        return peekFlag || iterator.hasNext();




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