Flatten 2D Vector — Python

# Time:  O(1)
# Space: O(1)

from collections import deque


class Vector2D(object):

    def __init__(self, vec2d):
        """
        Initialize your data structure here.
        :type vec2d: List[List[int]]
        """
        self.stack = deque((len(v), iter(v)) for v in vec2d if v)

    def next(self):
        """
        :rtype: int
        """
        length, iterator = self.stack.popleft()
        if length > 1:
            self.stack.appendleft((length-1, iterator))
        return next(iterator)

    def hasNext(self):
        """
        :rtype: bool
        """
        return bool(self.stack)