equiLeader :: [Int] -> Int
equiLeader a = length (filter isEqui (zip3 [0 ..] a prefixLeaderCounts))
  where
    n = length a

    (size, value) = foldl step (0, 0) a
    step (sz, val) v
      | sz == 0   = (1, v)
      | val /= v  = (sz - 1, val)
      | otherwise = (sz + 1, val)

    candidate   = if size > 0 then value else -1
    leaderCount = length (filter (== candidate) a)
    leader      = if leaderCount > n `div` 2 then candidate else -1

    prefixLeaderCounts = scanl1 (+) [if v == leader then 1 else 0 | v <- a]

    isEqui (k, _, lLeaderCount) =
      let leftHalf     = (k + 1) `div` 2
          rightHalf     = (n - k - 1) `div` 2
          rLeaderCount = leaderCount - lLeaderCount
      in  lLeaderCount > leftHalf && rLeaderCount > rightHalf