Suppose there are $$$x$$$ distinct characters, then we can simply transform $$$n$$$ into base $$$x$$$ to get the answer.

walk on A-ary trie by holding subtree size. Where A is Alphabet size

If this problem is about finding the nth lexicographical string, then the answer is character A n times, where A is the smallest lexicographical. I think they intended for the sequence to be sorted by length, and only then lexicographically. Then you can first find its length, denote it as L, then find nth lexicographical string of length L (n will be different after canceling out every string with size < L). I can share the code if you want