A simple way of encoding a message is the railfence cypher. The message is written downwards and diagonally on rails of an imaginary fence, moving down and up. The message, Howareyou? becomes

H . . . r . . . u .
. o . a . e . o . ? 
. . w . . . y . . .

Write a method that receives a string and that returns a two-dimensional array of strings, placing the characters as indicated in the preceding diagram. Each array element should either be null (represented by . above) or a string of length 1.