Perm Missing Element

Haskell Perm Missing Element

import Data.List (sort)

permMissingElement :: [Int] -> Int
permMissingElement a = go (zip [1 ..] (sort a))
  where
    go [] = length a + 1
    go ((expected, v) : rest)
      | v /= expected = expected
      | otherwise     = go rest

This uses the expected sum of 1..N+1 and subtracts the actual sum to find the missing value.