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The main focus here was optimizing the pandigital testing. The rest is just looping over every possible number.
We can optimize the outer loop a little bit if we look at the possible multiplicands. There are only 2 possibilities if we need 9 digits in our calculation:

• 1-digit number * 4-digit number = 4-digit number
• 2-digit number * 3-digit number = 4-digit number