#腾讯笔试#8.23  腾讯笔试第二题为什么回溯一个都过不了import sysclass Solution:    def subset(self,s,n):        self.res = []        partial = ""        self.used = [False] * len(s)        self.backtracking(partial,s)        self.res.sort()        return self.res[n-1]    def backtracking(self,partial,s):        if partial:            self.res.append(partial)        if len(partial) == len(s):            return        else:            for i in range(len(s)):                if not self.used[i]: if not partial or ord(s[i]) >= ord(partial[-1]): if i > 0 and s[i] == s[i-1] and not self.used[i - 1]:                            continue                        self.used[i] = True                        partial+=s[i]                        self.backtracking(partial,s)                        partial = partial[:-1]                        self.used[i] = Falsea = input().split()s = a[0]n = int(input().split()[0])print(str(Solution().subset(s,n)))