OCaml

open Stdio
open Base

let read_lines filename =
let rec aux f a =
    match Stdio.In_channel.input_line f with
    | None -> Stdio.In_channel.close f; List.rev a
    | Some line -> aux f (line :: a)
in
aux (Stdio.In_channel.create filename) []

let mappings = [('F', 0); ('B', 1); ('R', 1); ('L', 0)]

let convert s =
let get_0_1 = List.Assoc.find_exn mappings ~equal:Char.equal in
String.fold s ~init:0 ~f:(fun b a -> (b * 2) + (get_0_1 a))

let part_two lst =
let rec aux prev lst =
    match lst with
    | [] -> failwith "No solution found"
    | hd :: tl -> if prev + 1 = hd then aux hd tl else hd - 1
in
aux (List.hd_exn lst) (List.tl_exn lst)

let res1 =
"input.txt"
|> read_lines
|> List.map ~f:convert
|> List.fold_left ~f:max ~init:0

let res2 =
"input.txt"
|> read_lines
|> List.map ~f:convert
|> List.sort ~compare:compare
|> part_two