upon further exploration, it's because I change the argument arrays, and this makes the unit test expected calculation misbehave. Either the unit tests should be fixed (to at least display a meaningful error in that case...), or the instructions clarified.

Is there some issue with the random tests in ruby? I keep getting failures because it expects an empty array when I return an interleaved array. For example, in one of the random tests, the input was:
3 <<<<
t k 1 0 c l y <<<<
q x 5 <<<<
l 8 o s g a v <<<<
m v <<<<
j k s v r <<<<
b 4 s r f f 3 r <<<<
4 z 0 6 <<<<
<<<<
(each row is an array)
and I got the error message:
Expected: [], instead got: [3, "t", "q", "l", "m", "j", "b", 4, nil, nil, "k", "x", 8, "v", "k", 4, "z", nil, nil, 1, 5, "o", nil, "s", "s", 0, nil, nil, 0, nil, "s", nil, "v", "r", 6, nil, nil, "c", nil, "g", nil, "r", "f", nil, nil, nil, "l", nil, "a", nil, nil, "f", nil, nil, nil, "y", nil, "v", nil, nil, 3, nil, nil, nil, nil, nil, nil, nil, nil, "r", nil, nil]

Small spelling fix: at the second line, fix "buting" to "buying".

Also, not an issue per se, but I think it'll be good to add using System and/or using System.Linq to the initial code.