Stack a certain vectors of a matrix vertically

I have a hypothetical matrix say a =

     1     2     3
     2     4     5
     3     5     6

in one command, how I can stack the first two columns vertically to have a=[1 2 3 2 4 5]'

[Niveen El Zayat]

I got it I may use linear index so the answer is a(1:6)'

[the cyclist]

You could make this an answer (rather than a comment on the question).

[Niveen El Zayat]

I left it as a comment because i am not quite satisfied with this answer, I leave the space to some one give me a more generic answer fit any dimension

Answer 1

[Guillaume]

As you've figured out for a 3xN matrix, a(1:6)' would work. For an arbitrary sized MxN matrix:

reshape(a(:, 1:2), [], 1)

[Niveen El Zayat]

thanks, this is the answer I was looking for, because the above one is very simple hypothetical example my main code is in terms of large dimension and i did try the reshape command but i failed many time