Rows, columns, and rotation

Rows and columns are different axes. The exam's 2D traps are all axis confusions:

• g[r][c] — row first, always. g[0] is the first ROW, not the first column.
• g.length is the row count; g[0].length is the column count. Rectangular grids can have g.length != g[0].length — a rotation changes which one is which.
• Row-major traversal (for r, then for c) visits in storage order; column-major (for c, then for r) is the one FRQs ask for when they say "process column by column".

The 90° clockwise rotation is the canonical axis exercise: out[c][rows - 1 - r] = g[r][c]. Derive it once slowly: the old row r becomes the new column (rows-1-r) counted from the right; the old column c becomes the new row c. Verify on a 2×3 before you trust it.