class CombinationIterator:
    def __init__(self, characters: str, combinationLength: int):
        self.cs = list(characters)
        self.len = combinationLength
        self.cur = [i for i in range(self.len)]

    def next(self) -> str:
        result = "".join(self.cs[i] for i in self.cur)

        max_allowed_idx = len(self.cs) - 1
        while self.cur:
            self.cur[-1] += 1
            if self.cur[-1] <= max_allowed_idx:
                break
            self.cur.pop()
            max_allowed_idx -= 1

        if not self.cur:
            self.cur = None
        else:
            while len(self.cur) < self.len:
                self.cur.append(self.cur[-1] + 1)

        return result

    def hasNext(self) -> bool:
        return self.cur is not None