Given two words (start and end), and a dictionary, find the length of shortest transformation sequence from start to end, such that only one letter can be changed at a time and each intermediate word must exist in the dictionary. For example, given:
start = "hit"
end = "cog"
dict = ["hot","dot","dog","lot","log"]
One shortest transformation is "hit" -> "hot" -> "dot" -> "dog" -> "cog", the program should return its length 5.
