Problem $1\mathrm{.}4(10$ points$)$ : In image processing, sometimes it is necessary to convert an image $($a $2$D array of pixels$)$ to a $1$D array of pixels. If a is a $2D$ array of size MxN$,$ we would like to convert it to a $1$D array $($in row order$).($For example. If you have a multidimensional array of $34($three rows of four columns$)$ I would want to move the data, row by row, into a single array of length $12).$ Make sure your code handles the general case and does not just show the example above. Complete the code below.

public static int$[]$ flatten$($int$[][]$ a$)[$

if $($a$.$length$==0)$ return null;

int rows $=a.$ length;

int cols$=a[].$ length;

$//$Fill in the code here

return tmp;

$\}$