Concatenate columns

Dear forum,

I'm looking for a easy way to concatenate columns.

....A B C
1 1 4
2 2 5
3 3 6
4
5
6

I want to get the data of A and B into C:
C: 1 2 3 4 5 6

I have 123 colums of data which need to be merged into a single one.

Thanks for any help!