A happy string is a string that:
\n\n['a', 'b', 'c'].s[i] != s[i + 1] for all values of i from 1 to s.length - 1 (string is 1-indexed).For example, strings "abc", "ac", "b" and "abcbabcbcb" are all happy strings and strings "aa", "baa" and "ababbc" are not happy strings.
\n\nGiven two integers n and k, consider a list of all happy strings of length n sorted in lexicographical order.
Return the kth string of this list or return an empty string if there are less than k happy strings of length n.
\n
Example 1:
\n\n\nInput: n = 1, k = 3\nOutput: "c"\nExplanation: The list ["a", "b", "c"] contains all happy strings of length 1. The third string is "c".\n\n\n
Example 2:
\n\n\nInput: n = 1, k = 4\nOutput: ""\nExplanation: There are only 3 happy strings of length 1.\n\n\n
Example 3:
\n\n\nInput: n = 3, k = 9\nOutput: "cab"\nExplanation: There are 12 different happy string of length 3 ["aba", "abc", "aca", "acb", "bab", "bac", "bca", "bcb", "cab", "cac", "cba", "cbc"]. You will find the 9th string = "cab"\n\n\n
\n
Constraints:
\n\n1 <= n <= 101 <= k <= 100一个 「开心字符串」定义为：
\n\n['a', 'b', 'c'].1 到 s.length - 1 之间的 i ，满足 s[i] != s[i + 1] （字符串的下标从 1 开始）。比方说，字符串 "abc"，"ac"，"b" 和 "abcbabcbcb" 都是开心字符串，但是 "aa"，"baa" 和 "ababbc" 都不是开心字符串。
\n\n给你两个整数 n 和 k ，你需要将长度为 n 的所有开心字符串按字典序排序。
请你返回排序后的第 k 个开心字符串，如果长度为 n 的开心字符串少于 k 个，那么请你返回 空字符串 。
\n\n
示例 1：
\n\n输入：n = 1, k = 3\n输出："c"\n解释：列表 ["a", "b", "c"] 包含了所有长度为 1 的开心字符串。按照字典序排序后第三个字符串为 "c" 。\n\n\n
示例 2：
\n\n输入：n = 1, k = 4\n输出：""\n解释：长度为 1 的开心字符串只有 3 个。\n\n\n
示例 3：
\n\n输入：n = 3, k = 9\n输出："cab"\n解释：长度为 3 的开心字符串总共有 12 个 ["aba", "abc", "aca", "acb", "bab", "bac", "bca", "bcb", "cab", "cac", "cba", "cbc"] 。第 9 个字符串为 "cab"\n\n\n
示例 4：
\n\n输入：n = 2, k = 7\n输出：""\n\n\n
示例 5：
\n\n输入：n = 10, k = 100\n输出："abacbabacb"\n\n\n
\n\n
提示：
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