Give a word and return a rank, however, the word system order is like this:
We only consider those letters are in lexicographical order. (each character is smaller than next character)
Only lower letters and
a => rank 1
b => rank 2
...
z => rank 26
ab => rank 27
...
az
bc
...
abcde
abcdf
...
abcdefghijklmnopqrstuvwxyz

Do you guys have some ideas?