How to merge two lists with different dimensions

Question

I have two list, just like

a={a1,a2,a3,a4};
b={b1,b2,b3,b4,b5};

I want merge them and save to excel, as they have different dimention, so I want it have the following format.

{{a1,b1},{a2,b2},{a3,b3},{a4,b4},{null,b5}}

and in the excel it will leave blank or fill some special words in the miss information position.

Answer 1

You can use Transpose + PadRight:

Transpose @ PadRight[{a,b}, Automatic, Null]

{{a1, b1}, {a2, b2}, {a3, b3}, {a4, b4}, {Null, b5}}