How can I delete certain rows according to two columns which have symmetricl values in data.table in R?

Question

For example, I have a table as follows:

DT <- data.table(
  A = c(1,1,1,2,2,2,3,3,3), 
  B = c(1,2,3,1,2,3,1,2,3),
  key = \"A\"
)

I wand to

Answer 1

if you only have two columns, then you could do:

 unique(do.call(function(A,B)data.table(A=pmin(A,B),B=pmax(A,B)),DT))
   A B
1: 1 1
2: 1 2
3: 1 3
4: 2 2
5: 2 3
6: 3 3