Running On matrix by diagonals

I am looking for an algorithm that can collect the elements of a matrix in the following way

It can be a non square matrix too

So the answer would be [a41,a42,a31,a43,a32,a21,a44,...]

Answer 1

[Guillaume]

One option:

[rows, cols] = size(A);
indices = (1:cols+rows)*rows - (0:rows-1)'*(rows+1);
indices = indices(indices > 0 & indices <= numel(A));
A(indices)

Require R2016b or later for the implicit expansion of the subtraction