# Time:  O(k), per operation
# Space: O(k)

import itertools


class CombinationIterator(object):

    def __init__(self, characters, combinationLength):
        """
        :type characters: str
        :type combinationLength: int
        """
        self.__it = itertools.combinations(characters, combinationLength)
        self.__curr = None
        self.__last = characters[-combinationLength:]

    def next(self):
        """
        :rtype: str
        """
        self.__curr = "".join(self.__it.next())
        return self.__curr
    
    def hasNext(self):
        """
        :rtype: bool
        """
        return self.__curr != self.__last


# Time:  O(k), per operation
# Space: O(k)
import functools


class CombinationIterator2(object):

    def __init__(self, characters, combinationLength):
        """
        :type characters: str
        :type combinationLength: int
        """
        self.__characters = characters
        self.__combinationLength = combinationLength
        self.__it = self.__iterative_backtracking()
        self.__curr = None
        self.__last = characters[-combinationLength:]
        
    def __iterative_backtracking(self):
        def conquer():
            if len(curr) == self.__combinationLength:
                return curr

        def prev_divide(c):
            curr.append(c)
        
        def divide(i):
            if len(curr) != self.__combinationLength:
                for j in reversed(xrange(i, len(self.__characters)-(self.__combinationLength-len(curr)-1))):
                    stk.append(functools.partial(post_divide))
                    stk.append(functools.partial(divide, j+1))
                    stk.append(functools.partial(prev_divide, self.__characters[j]))
            stk.append(functools.partial(conquer))

        def post_divide():
            curr.pop()
            
        curr = []
        stk = [functools.partial(divide, 0)]
        while stk:
            result = stk.pop()()
            if result is not None:
                yield result

    def next(self):
        """
        :rtype: str
        """
        self.__curr = "".join(next(self.__it))
        return self.__curr
        
    def hasNext(self):
        """
        :rtype: bool
        """
        return self.__curr != self.__last


# Your CombinationIterator object will be instantiated and called as such:
# obj = CombinationIterator(characters, combinationLength)
# param_1 = obj.next()
# param_2 = obj.hasNext()