Matrices & grids

A matrix (2D grid) is an array of arrays addressed by [row][col]. Many grid problems are implicit graphs: each cell is a vertex whose neighbours are the cells up, down, left, and right (sometimes the diagonals), so BFS and DFS apply directly without building an explicit graph.

Reach for grid BFS/DFS for flood-fill, island-counting, and shortest-path-in-a-maze. Watch for in-place transforms (rotate, set-zeroes, spiral) and, above all, careful bounds checking at the edges.