It is possible to use a recursion strategy without using any sudoku-specific solver techniques, and without having to worry about time constraints.

Hi,

It's a numeral system with a base equal to 27.

Decimal is the system you use everyday to count; it's a numeral system with a base equal to 10, it has 10 different digits: 0 to 9.

hexadecimal is another system, with a base equal to 16. It has 16 different digits: 0 to 9, plus A to F. We use it a lot to represent the value of a byte.

0 to 9 in hexadecimal is equal to 0 to 9 in decimal. Then, A is equal to 10, B is equal to 11, C is equal to 12, D is equal to 13, E is equal to 14, and F is equal to 15.

So:

0 in hexadecimal is equal to 0 in decimal

1 in hexadecimal is equal to 1 in decimal

But, 10 in hexadecimal is NOT equal to 10 in decimal

10 in hexadecimal is equal to 16 in decimal

For exactly the same reason, "A " in our base 27 numeral system is equal to 27 in decimal.

If this is still confusing for you, I suggest you read more about numeral systems by following the Wikipedia links I provided.

Cheers

if you're exceeding the call stack size with these test cases, your algorithm is inefficient or incorrect