Given a square matrix, rotate it 90 degrees clockwise, in place: the element at matrix[i][j] moves to result[j][n-1-i]. Building a brand new matrix and placing each element at its rotated destination works, but it costs a full second matrix's worth of memory. A 90° clockwise rotation can be decomposed into two simpler, in-place operations: a transposeTransposeFlipping a matrix across its main diagonal — matrix[i][j] and matrix[j][i] swap places. (flip across the main diagonal), followed by reversing every row (a horizontal flip). Composing those two flips is exactly the same as rotating 90° clockwise — no extra matrix required.