Matrix & Grid

Rows, columns and neighbours — the grid problems interviews keep drawing.

1. 1Grid TraversalRow first, column second, and always check the edge.4m
2. 2Spiral OrderFour walls that close in after every pass.5m
3. 3Rotate In PlaceMirror the diagonal, then flip each row.5m
4. 4Number of IslandsCount a blob once, then erase it so it cannot count twice.5m
5. 5Flood FillSpread while the colour matches, stop the moment it does not.4m