問題は http://nabetani.sakura.ne.jp/hena/ord7selectchair/
解答リンク集は http://qiita.com/items/4364285801d1c9f370a1

面白そうな問題だったので、Haskell で実装してみました。
座席の両端に座れない空席を置いて判定を単純化してみようと頑張ってみまし
た。

off-line07.hs

import Data.Char
import System.Environment

main :: IO ()
main = do arg <- getArgs
          let (n, str) = getData (arg !! 0)
          print $ solve n str

getData :: String -> (Int, String)
getData cs = (read a, tail b)
  where (a, b) = break (== ':') cs

solve :: Int -> String -> String
solve n cs = init $ tail $ foldl f (replicate (n + 2) '-') cs
  where f seat c = if (isUpper c) then sit c seat else rm (toUpper c) seat

sit :: Char -> String -> String
sit c cs = as ++ (c : bs)
  where
    (as, _ : bs) = splitAt (search cs) cs
    search cs@(_ : cs')
      | take 2 cs == "--" = search' 0 ( 1, -1) cs
      | otherwise         = search' 1 (-1, -1) cs'
    -- search' c (i1, i2) cs
    --  c : index counter
    --  i1, i2 : 「条件 2」と「条件 3」に適合する index
    search' _ (i1, i2) "-" = if i1 < 0 then i2 else i1
    search' c (i1, i2) cs@(s : cs')
      | take 3 cs == "---"              = c + 1
      | (i1 < 0) && (take 2 cs == "--") = search' (c + 1) ( c, i2) cs'
      | (i2 < 0) && (s == '-')          = search' (c + 1) (i1,  c) cs'
      | otherwise                       = search' (c + 1) (i1, i2) cs'

rm :: Char -> String -> String
rm c cs = as ++ ('-' : bs)
  where (as, _ : bs) = break (== c) cs