Trial and error: test one candidate to contradiction
Trial and error assumes one candidate temporarily, follows only forced moves, and checks whether the branch creates a contradiction.
Key idea
A contradicted assumption is false, so the other candidate is true. The method is rigorous only when every propagation step is recorded and reversible.
See the move, step by step
Test candidate 2 at R1C5 and follow the consequences down column 5.
Step 1 of 4
Choose a bivalue cell
R1C5 contains 2 and 8. Column 5 gives a short, auditable path for testing candidate 2.
Step 2 of 4
Mark the assumption
Assume R1C5 is 2. Forced singles then place 7 at R7C5 and 3 at R8C5.
Step 3 of 4
Stop at the first contradiction
R9C5 loses 2, 3, and 7 and has no legal candidate. The branch is impossible.
Step 4 of 4
Undo the branch and keep its proof
Remove 2 from R1C5 and place 8. All temporary branch placements are discarded.
Try it on today's puzzle
Use this only after systematic techniques stall. Change one assumption at a time, record every forced consequence, and fully undo the branch before continuing.
Play today