[leetcode284]Peeking Iterator

问题描述：

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.

算法：

将关注点着眼于数据，直接对数据（iterator, top, isPeeked）操作。
通过引入顶端元素top，和判断数据isPeeked对迭代器的状态进行判断。
当迭代未开始时：top=None， isPeeked = False
当迭代终止时：top=None， isPeeked = False
中间状态为：top is not None， isPeeked = True
在函数中，要判断状态并进行转换。

# 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.top = None    # for next
        self.isPeeked = False   # is peeked?


    def peek(self):
        """ Returns the next element in the iteration without advancing the iterator. :rtype: int """
        if self.isPeeked:
            return self.top
        else:
            if self.iterator.hasNext():
                self.top = self.iterator.next()
                self.isPeeked = True
                return self.top
            else:
                self.top = None
                self.isPeeked = False
                return self.top



    def next(self):
        """ :rtype: int """
        if self.isPeeked:
            if self.iterator.hasNext():
                value = self.top
                self.top = self.iterator.next()
                return value
            else:
                value = self.top
                self.top = None
                self.isPeeked = False
                return value
        else:
            if self.iterator.hasNext():
                value = self.iterator.next()
                self.top = self.iterator.next()
                self.isPeeked = True
                return value
            else:
                self.top = None
                self.isPeeked = False
                return None



    def hasNext(self):
        """ :rtype: bool """
        if self.iterator.hasNext():
            return True
        else:
            return self.top 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].