Problem Statement

You have a gene sequence represented by an 8-character string, containing only 'A', 'C', 'G', or 'T' characters. To change from one gene sequence to another, you can mutate one character at a time.

For example, "AAAAACCC" --> "AAAACCCC" is one mutation.

Each intermediate sequence must be a valid gene in the provided list (bank).

The gene bank bank records all the valid gene mutations. A gene must be in bank to make it a valid gene string.

Given the two gene strings startGene and endGene and the gene `bank