aimperiali
commented
6 years ago To expand on the existing method, the following pseudocode would fall between the if and else statements noted above. This is intended to check the minimum number of cells required to know if the player is following a solution different valid solution for the given board constraints. This method makes the assumption that alternate solutions arise in the form of the corners of a rectangle that can be swapped around without violating the board constraints displayed.
User selects a cell (lets name it cell c0)
Cell is checked against the solution array
If cell is different than the corresponding cell in the original solution (called solution1):
for every empty cell (called cA) in the same column (providing cA != c0):
for every empty cell (called cB) in the same row as cA:
remaining corner of this rectangle can be called cell cC
create a temporary solution where the contents of cell cA are switched with cell c0 and the contents of cell cB are switched with cell cC (called solution2)
check if solution2 has the same board constraints as solution1
if solution2 is as valid as solution1:
replace solution1 with solution 2
mark cell c0 as correct
break (stop checking)
else:
mark cell c0 as incorrect
(keep checking)
josephcsible
m-sterling
liouh
bertek41
and I can't even start. Seed is 
When a cell is selected, the validity of that solution is checked against a single predetermined solution. This does not take into account scenarios where multiple solutions exist [See image below]
One solution would be to add an else if statement between the if and else statements shown above. This else if statement would check if the selected cell contradicts the rest of the board constraints. This solution is not enough on its own because the stored solution would have to be updated to reflect the new solution required.
Because of this, I would focus deriving an algorithm that can efficiently check if the current state of the board can reach a solved state or if there is bound to be a contradiction to the board constraints. This algorithm could then be used in place of the stored-solution-check noted above. I'll be giving this some more thought in my spare time.