An inefficient solution to the 'word ladder ii' problem, and it is too slow to pass the large data set. Tried several slightly different approach to mark node as visited during BFS: 1) track all visited nodes's string value, which will lead to a solution that cannot find all solutions 2) use a linked list to track all the words in the path, too slow 3) use a linked hash set to track all the words in the path, still too slow