Row-Column Scanning
Systematically scan each row and column to find cells where only one valid placement remains.
IntermediateThe Scanning Technique
Row-column scanning is the systematic process of checking each row and column to count the number of valid queen placements. If a row (or column) has only one valid cell remaining — after accounting for region constraints, other queens, and adjacency — that cell must contain a queen.
How to Scan Efficiently
Go through each row from top to bottom. For each row, check every cell: is it in a region that already has a queen? Is it adjacent to an existing queen? Is its column already occupied? If only one cell passes all three checks, place a queen there. Then rescan, because the new queen may have created new forced placements.
Combining with Region Logic
Scanning is most powerful when combined with region awareness. If you're scanning row 5 and find two valid cells, check which regions they belong to. If one region already has fewer options elsewhere, that can break the tie. Row-column scanning and region elimination work together like a two-pronged attack.
When Scanning Isn't Enough
In harder puzzles, no row or column may have a unique solution after basic scanning. That's when you need advanced techniques like region confinement and exclusion logic. But always start with scanning — it's the fastest way to make progress.