let input = [| -1; 3; -4; 5; 1; -6; 2; 1 |]

​

let findEquilibrium (input : int[]) =

  let front = Array.copy input

  let rear  = Array.copy input

​

  for i = 1 to input.Length-1 do

    front.[i] <- front.[i] + front.[i-1]

​

  for i in input.Length-2 .. -1 .. 0 do

    rear.[i] <- rear.[i] + rear.[i+1]

​

  { 0..input.Length-1 }

  |> Seq.filter (fun idx -> front.[idx] = rear.[idx])

  |> Seq.toArray

let findEquilibrium2 (input : int[]) =

  let sum = ref (input |> Seq.sum)

​

  seq {

    let acc = ref 0

​

    for i = 0 to input.Length-1 do

      let n = input.[i]

      if !sum - n = !acc then yield i

      sum := !sum - n

      acc := !acc + n

  }

  |> Seq.toArray

​

​

​

findEquilibrium input |> printfn "%A"

​

findEquilibrium2 input |> printfn "%A"