<p>A <strong>happy string</strong> is a string that:</p>
<ul>
<li>consists only of letters of the set <code>['a', 'b', 'c']</code>.</li>
<li><code>s[i] != s[i + 1]</code> for all values of <code>i</code> from <code>1</code> to <code>s.length - 1</code> (string is 1-indexed).</li>
</ul>
<p>For example, strings <strong>"abc", "ac", "b"</strong> and <strong>"abcbabcbcb"</strong> are all happy strings and strings <strong>"aa", "baa"</strong> and <strong>"ababbc"</strong> are not happy strings.</p>
<p>Given two integers <code>n</code> and <code>k</code>, consider a list of all happy strings of length <code>n</code> sorted in lexicographical order.</p>
<p>Return <em>the kth string</em> of this list or return an <strong>empty string</strong> if there are less than <code>k</code> happy strings of length <code>n</code>.</p>
<strong>Explanation:</strong> The list ["a", "b", "c"] contains all happy strings of length 1. The third string is "c".
<strong>Explanation:</strong> 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 9<sup>th</sup> string = "cab"
