Solution in haskell for the phone list problem from open.kattis.com

phoneList.hs

import Control.Monad (replicateM)

data Tree a = EmptyTree | Node a [Tree a] deriving (Show, Eq)

label :: Tree a -> a
label (Node a _) = a

subforest :: Tree Char -> [Tree Char]
subforest (Node _ subforest) = subforest

countLeaves :: Tree Char -> Int
countLeaves t =
    if (subforest t) == []
        then 1
    else sum $ map countLeaves (subforest t)

insertForest :: String -> [Tree Char] -> [Tree Char]
insertForest "" forest = forest
insertForest (d:ds) [] = [Node d (insertForest ds [])]
insertForest number@(d:ds) (t:ts) =
    if d == label t
        then (Node (label t) (insertForest ds (subforest t))):ts
    else t:(insertForest number ts)

insertNumbers :: [String] -> [Tree Char] -> [Tree Char]
insertNumbers [] forest = forest
insertNumbers (n:ns) forest = insertNumbers ns (insertForest n forest)

performTestCase :: IO String
performTestCase = do
    numberOfLines <- getLine
    numbers <- replicateM (read numberOfLines) getLine
    let forest = insertNumbers numbers []
    let count = sum $ map countLeaves forest
    if count == length numbers then return "YES" else return "NO"

main = do
    testCases <- getLine
    testResults <- replicateM (read testCases) performTestCase
    mapM putStrLn testResults