Functional programing function -

how write functional program union of 2 lists or sets?

it depends on language, generally, there'll recursive solution involving traversal of sets identifying shared elements.

e.g. in haskell on native data.set type,

union :: ord => set -> set -> set union tip t2  = t2 union t1 tip  = t1 union t1 t2 = hedgeunion (const lt) (const gt) t1 t2  hedgeunion _     _     t1 tip   = t1 hedgeunion cmplo cmphi tip (bin _ x l r)   = join x (filtergt cmplo l) (filterlt cmphi r) hedgeunion cmplo cmphi (bin _ x l r) t2   = join x (hedgeunion cmplo cmpx l (trim cmplo cmpx t2))            (hedgeunion cmpx cmphi r (trim cmpx cmphi t2))       cmpx y  = compare x y 

or more simply, lists:

unionby                 :: (a -> -> bool) -> [a] -> [a] -> [a] unionby eq xs ys        =  xs ++ foldl (flip (deleteby eq)) (nubby eq ys) xs