complex query

I am trying to get following result set out of single column. In a column when it hit a particular value in columnA it will change column B value 

Table A

Column A

-----------------

111

AAA

BBB

CCC

AAA

222

XXX

AAA

DDD

Result

Column A      COLUMNB

AAA                111

BBB                111

CCC               111

AAA                111

XXX                 222

AAA                 222

DDD                222