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

Find the k-th lexicographically smallest length-n string over {a,b,c} with no two adjacent characters equal, or return empty if fewer than k exist. Use combinatorial counting (total = 3 * 2^{n-1}) to construct the answer character-by-character by skipping blocks of size 2^{remaining-1} rather than enumerating all strings.