This problem can be solved by equations, for example
we make equations for the input:

3
1 2 -1
1 2 3 -1
3 -1

whe construct the matrix

1 1 0 = 1
1 1 0 = 1
0 1 1 = 1

since 1st and 2nd equations are the same, it means that
you are free to choose the value for the 3rd variable,
but in fact if you choose its value arbitrarily,
answer length can be changed, two possible answers
are 1,2 and 3, but only 3 is the right answer, because
it's the shortest. What to do with that?

this test is incorrect, because 1 combine 3 = 2