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

getHappyString.js

/**
 * @param {number} n
 * @param {number} k
 * @return {string}
 */
var getHappyString = function(n, k) {
    // Helper function to perform backtracking and generate all happy string
    function backtrack(current, result) {
        // If the current string has reached the desired length, add it to the result list
        if (current.length === n) {
            result.push(current);
            return;
        }
        // Iterate through the characters 'a', 'b', 'c'
        for (let char of ['a', 'b', 'c']) {
            // Only add the character if it's different from the last character in the current string
            if (current.length === 0 || current[current.length - 1] !== char) {
                backtrack(current + char, result);
            }
        }
    }

    // List to store all happy strings
    let result = [];
    // Start backtracking with an empty string
    backtrack("", result);

    // If the list has fewer than k happy strings, return an empty string
    if (result.length < k) {
        return "";
    }
    // Return the k-th happy string (1-indexed)
    return result[k - 1];
};