for encoding that {a,b,c,...,z}<->{1,2,3,...,26}
if given a list of digit e.g. [1,2,3], this may represent {1,2,3}->{a,b,c} or {12,3}->{l,c} or {1,23}->{a,w}. so there are 3 possible interpretations for list [1,2,3]
so, given a list of digit, calculate the number of possible interpretations for the list.