边工作边刷题：70天一遍leetcode: day 56-1

Peeking Iterator

要点：想成cache会比较容易理解，要进一步要考虑什么时候put to cache和什么时候invalidate cache，

• put to cache：peek的时候，如果没在cache中，因为iterator已经移动，直到下一次next，都要在cache中找。
• invalidate cache：next的时候如果在cache中，返回cache同时要invalidate，因为之后无论call peek or next都是要找到当前的下一个值，当前值没用了。

错误点：

• 在peek里call了2次iterator.next()，put to cache的时候call一次就够了

# 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.cache = None

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

    def next(self):
        """
        :rtype: int
        """
        if self.cache:
            ret = self.cache
            self.cache = None
            return ret

        return self.iterator.next()
        

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