Count all solutions sequentially and in parallel

nqueens4.fs

// Place the first queen beforehand for parallel execution.
let countParallel1 size =
    let boards = Array.init size (fun _ -> new Board(size)) 
    for i in 0..size-1 do
        boards.[i].placeQueen(i)

    boards |> Array.Parallel.map (fun b -> b.countSolutions()) 
           |> Array.sum

// Place two first queens beforehand for parallel execution.
let countParallel2 size =
    let n = (size-1)*(size-2)
    let boards = Array.init n (fun _ -> new Board(size)) 
    let mutable count = 0
    for i in 0..size-1 do
        for j in 0..size-1 do
            if abs(i-j) > 1 then // safe to place two queens together
                boards.[count].placeQueen(i)
                boards.[count].placeQueen(j)
                count <- count+1
    
    boards |> Array.Parallel.map (fun b -> b.countSolutions()) 
           |> Array.sum