leetcode1415. The k-th Lexicographical String of All Happy Strings of Length n

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【leetcode1415. The k-th Lexicographical String of All Happy Strings of Length n】题目如下：

A happy string is a string that:

consists only of letters of the set [‘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.
Given 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.

Example 1:

Input: n = 1, k = 3 Output: "c" Explanation: The list ["a", "b", "c"] contains all happy strings of length 1. The third string is "c".

Example 2:

Input: n = 1, k = 4 Output: "" Explanation: There are only 3 happy strings of length 1.

Example 3:

Input: n = 3, k = 9 Output: "cab" Explanation: 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"

Example 4:

Input: n = 2, k = 7 Output: ""

Example 5:

Input: n = 10, k = 100 Output: "abacbabacb"

Constraints:

1 < = n < = 10

1 < = k < = 100

解题思路：n和k都很小，最简单的方法是把所有符合条件的string都求出来，排序后可得到结果。
代码如下：

class Solution(object): def getHappyString(self, n, k): """ :type n: int :type k: int :rtype: str """ queue = [‘a‘,‘b‘,‘c‘]l = [] while len(queue) > 0: item = queue.pop(0) if len(item) == n: l.append(item) continue if item[-1] == ‘a‘: queue.append(item + ‘b‘) queue.append(item + ‘c‘) elif item[-1] == ‘b‘: queue.append(item + ‘a‘) queue.append(item + ‘c‘) elif item[-1] == ‘c‘: queue.append(item + ‘a‘) queue.append(item + ‘b‘)l.sort() if len(l) < k: return ‘‘ return l[k-1]