How to print columns one after the other in bash?

Question

Is there any better methods to print two or more columns into one column, for example

input.file

AAA    111
BBB    222
CCC    333

o

Answer 1

Since the order is of no importance:

$ awk 'BEGIN {RS="[ \t\n]+"} 1' file
AAA
111
BBB
222
CCC
333