Rotate Image

1. The obvious way

   Build a new matrix and copy each cell to its rotated place: O(n²) extra space.

2. The key insight

   Transpose: for each pair with column > row, swap matrix[row][column] and matrix[column][row]. Then reverse every row. Both steps are in place.

3. The algorithm
   1. Swap each cell above the diagonal with its mirror below.
   2. Now rows and columns are exchanged (transposed).
   3. Reverse every row.
   4. The matrix is rotated.

Watch it run

[[1,4,7],[2,5,8],[3,6,9]]

def rotate(matrix):
    size = len(matrix)
    for row in range(size):
        for column in range(row + 1, size):
            matrix[row][column], matrix[column][row] = matrix[column][row], matrix[row][column]   # flip along the diagonal
    for row in matrix:
        row.reverse()                                    # mirror each row

Now close this and write it again from memory, in the language you will use in the interview. Reattempt after 1 day, 7 days and about 30 days.