How to generate sequence numbers

Consider matrix A as follows:

A = [
1
1
1
2
2
2
2
2
3
3
4
4
5
];

I want to generate a sequence numbers and reset these numbers wherever the ID changed.

B = [
1  1
2  1
3  1
1  2
2  2
3  2
4  2
5  2
1  3
2  3
1  4
2  4
1  5
];

Answer 1

[Stephen23]

>> N = diff(find([1;diff(A)~=0;1]));
>> cell2mat(arrayfun(@(n)(1:n).',N(:),'uni',0))
ans =
     1
     2
     3
     1
     2
     3
     4
     5
     1
     2
     1
     2
     1

Answer 2

[Andrei Bobrov]

out = ones(numel(A)+1,1);
ii = [true;diff(A)~=0];
jj = diff(find([ii;1]));
out(ii) = out(ii)-[0;jj(1:end-1)];
out = cumsum(out(1:end-1));