1415. The k-th Lexicographical String of All Happy Strings of Length n

https://leetcode.com/problems/the-k-th-lexicographical-string-of-all-happy-strings-of-length-n

Description

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

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