Common Mistakes - Walkthrough

Mistake $1$: Wrong bit indexing. Decide: bit $0 = leftmost$ or rightmost column? Stay consistent. Mistake $2$: Forgetting to process cells in order. Column by column, row by row.

Don't jump around. Mistake $3$: Invalid initial/final states. Start with $mask = 0$ (nothing filled). End with $mask = 0$ (everything filled). Mistake $4$: Off-by-one in grid dimensions. Carefully index rows and columns from $0$ or $1$. Off-by-one errors are especially common. Test on 2x2 and 2x3 grids where you can verify by hand.