<h2 id="heading-url">URL</h2>
<p><a target="_blank" href="https://leetcode.com/problems/permutations/description/">Permutations - LeetCode</a></p>
<h2 id="heading-code">Code</h2>
<p><a target="_blank" href="https://github.com/syohex/dotnet-study/tree/master/fsharp/leetcode/problems/0046/main.fsx">https://github.com/syohex/dotnet-study/tree/master/fsharp/leetcode/problems/0046/main.fsx</a></p>
<pre><code class="lang-plaintext">let permute (nums: int list) : int list list =
    let rec permute' (nums: int[]) used acc len ret =
        if len = nums.Length then
            (List.rev acc) :: ret
        else
            nums
            |&gt; Array.fold
                (fun ret n -&gt;
                    if Set.contains n used then
                        ret
                    else
                        permute' nums (Set.add n used) (n :: acc) (len + 1) ret)
                ret

    permute' (Array.ofList nums) Set.empty [] 0 []
</code></pre>
<p>functional programming version</p>
<pre><code class="lang-plaintext">let rec permute2 (nums: int list) : int list list =
    let rec interleave n ns =
        match ns with
        | [] -&gt; [ [ n ] ]
        | h :: t -&gt; (n :: h :: t) :: (List.map (fun ns -&gt; h :: ns) (interleave n t))

    match nums with
    | [] -&gt; []
    | h :: [] -&gt; [ [ h ] ]
    | h :: t -&gt; permute2 t |&gt; List.fold (fun acc ns -&gt; (interleave h ns) @ acc) []
</code></pre>


## URL

[Permutations - LeetCode](https://leetcode.com/problems/permutations/description/)

## Code

[https://github.com/syohex/dotnet-study/tree/master/fsharp/leetcode/problems/0046/main.fsx](https://github.com/syohex/dotnet-study/tree/master/fsharp/leetcode/problems/0046/main.fsx)

```plaintext
let permute (nums: int list) : int list list =
    let rec permute' (nums: int[]) used acc len ret =
        if len = nums.Length then
            (List.rev acc) :: ret
        else
            nums
            |> Array.fold
                (fun ret n ->
                    if Set.contains n used then
                        ret
                    else
                        permute' nums (Set.add n used) (n :: acc) (len + 1) ret)
                ret

    permute' (Array.ofList nums) Set.empty [] 0 []
```

functional programming version

```plaintext
let rec permute2 (nums: int list) : int list list =
    let rec interleave n ns =
        match ns with
        | [] -> [ [ n ] ]
        | h :: t -> (n :: h :: t) :: (List.map (fun ns -> h :: ns) (interleave n t))

    match nums with
    | [] -> []
    | h :: [] -> [ [ h ] ]
    | h :: t -> permute2 t |> List.fold (fun acc ns -> (interleave h ns) @ acc) []
```

LeetCode 46. Permutations in F#

URL

Code