User Instructions
#
# Write a function, solve(formula) that solves cryptarithmetic puzzles.
# The input should be a formula like 'ODD + ODD == EVEN', and the
# output should be a string with the digits filled in, or None if the
# problem is not solvable.
#
# Note that you will not be able to run your code yet since the
# program is incomplete. Please SUBMIT to see if you are correct.