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2D Array / Matrix
Rows × columns of contiguous data. Traversal patterns (spiral, diagonal, transpose) and grid-graph algorithms live here.
What to know
- Row-major traversal is dramatically faster than column-major due to cache lines.
- In-place rotation = transpose + reverse each row.
- Many DP problems (edit distance, unique paths) are matrices where each cell depends on neighbors.
In the wild: Image pixels, spreadsheets, game boards, ML tensors before they grow more dimensions.
Algorithms to reach for
Spiral / diagonal traversal
O(rows · cols)Layer-by-layer boundary walking
Matrix rotation in place
O(n²)Rotate image 90° without extra memory
Staircase search
O(rows + cols)Search a row-and-column sorted matrix from a corner
Practice problems — with full guides
Each problem has its own page: progressive hints, how to approach it, and the full solution.