“and each of the nine 3x3 sub-grids (also known as blocks) contain all of the digits from 1 to 9.”

The problem is you're mutating the input.

I didn't see the response in the earlier post but since you confirm it's correct, another look shows the sub 3x3 grid is the problem. thx.

Expected 'false' on input of

1 2 3 4 5 6 7 8 9

2 3 1 5 6 4 8 9 7

3 1 2 6 4 5 9 7 8

4 5 6 7 8 9 1 2 3

5 6 4 8 9 7 2 3 1

6 4 5 9 7 8 3 1 2

7 8 9 1 2 3 4 5 6

8 9 7 2 3 1 5 6 4

9 7 8 3 1 2 6 4 5