def lengthOfLongestSubstringKDistinct(self, s, k):
        # write your code here
        if s == None or len(s) < k:
            print([])

        hash = {}
        ans = []
        word_set = set()
        j = 0
        for i in range(len(s)):
            if i > 0:
                hash[s[i - 1]] -= 1
                if hash[s[i - 1]] == 0:
                    del hash[s[i - 1]]
            while (j < len(s) and j - i + 1 <= k):
                if s[j] in hash:
                    hash[s[j]] += 1
                else:
                    hash[s[j]] = 1

                if j - i + 1 == k:
                    if len(hash) == k:
                        if s[i:(j+1)] not in word_set:
                            ans.append(s[i:(j+1)])
                            word_set.add(s[i:(j+1)])
                j += 1
        print(ans)