F# solution for water on land problem

waterCapacity.fs

let oneWayCapacity a =
    let mutable max = 0
    a |> List.map (fun e -> if e > max then max <- e; 0 else max - e)
    
let totalCapacity a =
    let aL = a |> oneWayCapacity
    let aR = a |> List.rev |> oneWayCapacity |> List.rev
    List.map2 min aL aR |> List.sum


let test a =
    a |> totalCapacity |>
    printfn "totalCapacity %A = %i" a
    
let () = 
    [1; 2; 1; 3; 5; 3; 1; 2; 4; 1] |> test
    [2; 5; 10; 7; 3; 8; 5; 2] |> test
    [2; 5; 1; 2; 3; 4; 7; 7; 6] |> test



(* Task description

calculate how much water * will be left
on the land after the rain for any given array
             _ _ 
            |7 7|_
   _        |    6|
  |5|* * * *|     |
  | |* * *|4      |
 _| |* *|3        |
 2  |*|2          |
     1

[2,5,1,2,3,4,7,7,6] gives answer of 10

*)