rotates the given 2-D array by 90 degrees.
y = rot90 (x, k)
a 2D array.
an integer which specifies how many 90-degree rotations are to be applied.
Return a copy of x with the elements rotated counterclockwise in 90-degree increments. The second argument is optional, and specifies how many 90-degree rotations are to be applied (the default value is 1). Negative values of k rotate the matrix in a clockwise direction.