Construct a variable that conditionally takes a certain value until another condition is met.

So I have a panel dataset with data on conflicts for which I want to identify the post-conflict years. So I constructed a variable myself, which codes a transition from conflict to peace with "3". Whenever the values for a new country begin, I coded that same variable with NA. So what I want to do now is to create a new binary variable which identifies post-conflict years with a 1 and conflict years and never conflict with 0. For that I would have to assign every year, following a 3 in the transition variable with a 1 until there is an NA in the same column. As follows:

Country Year transition post-conflict

Afghanistan   1994   0  0
Afghanistan   1995   0  0
Afghanistan   1996   3  1
Afghanistan   1997   2  1
Afghanistan   1998   2  1
Albania       1994   NA 0
Albania       1994   2  0

Thanks a lot for you guys' help

Best wishes,