A happy string uses only a, b, and c, and no two neighboring letters are equal. List all happy strings of length n in alphabetical (lexicographical) order. Return the string at the 1-based position k, or the empty string "" if the list has fewer than k entries.
Example 1:
Input: n = 2
k = 4
Output: "bc"Explanation: The ordered list is ab, ac, ba, bc, ca, cb. Its fourth entry is bc.
Example 2:
Input: n = 2
k = 7
Output: ""Explanation: There are only six happy strings of length 2, so a seventh entry does not exist.
Example 3:
Input: n = 1
k = 2
Output: "b"Explanation: The one-letter list is a, b, c, so the second entry is b.
Constraints:
1 <= n <= 101 <= k <= 100