Haskell solution for CodeChef - Ambiguous Permutations

permutations.hs

import Control.Monad
import Data.List
import Data.Maybe
import System.Exit

main = do
    -- get the number for this case, and exit if it's equal to 0
    n <- liftM read getLine :: IO (Int)
    when (n == 0) exitSuccess
    
    -- read the input and calculate the answer
    rawInput <- liftM words getLine :: IO [String]
    let p = map read rawInput :: [Int]
    putStrLn $ solve p n
    
    -- recurse
    main
    
solve :: [Int] -> Int -> String
solve p n
    | p == inverse p n = "ambiguous"
    | otherwise        = "not ambiguous"

-- convert a list of numbers into its inverse permutation
inverse :: [Int] -> Int -> [Int]
inverse p n = f [1..n]
    where f []     = []
          f (x:xs) = (fromJust $ findIndex (==x) p) + 1 : f xs