In  cryptarithmetic puzzles, mathematical equations are written using letters.
Each letter can be a digit from 0 to 9, but no two letters can be the same.
Here is a sample problem:

SEND + MORE = MONEY

A solution to the puzzle is S=9, R=8,  O=0, M=1, Y=2, E=5, N=6, D=7

Write a program that finds solutions to the cryptarithmetic puzzle of:

TOO + TOO + TOO + TOO = GOOD

The simplest technique is to use a nested loop for each unique letter
(in this case T, O, G, D). The loops would systematically assign the
digits from 0-9 to each letter. For example, it might first try T=0,
O=0, G=0, D=0, then T=0, O=0, G=0, D=1, then T=0, O=0, G=0, D=2,
etc. up to T=9, O=9, G=9, D=9. In the loop body test that each
variable is unique and that the equation is satisfied. Output the
values for the letters that satisfy the equation.