Frog River One

Haskell Frog River One

import qualified Data.Set as Set

frogRiverOne :: Int -> [Int] -> Int
frogRiverOne x a = go (zip [0 ..] a) Set.empty
  where
    go [] _ = -1
    go ((k, i) : rest) seen
      | i <= x && not (Set.member i seen) =
          let seen' = Set.insert i seen
          in  if Set.size seen' == x then k else go rest seen'
      | otherwise = go rest seen

This tracks the earliest time each needed position appears and stops as soon as the frog can cross.