Given an array where each element is the price of a given stock on that index's day, choose a single day to buy a stock and a different day (in the future/later in the array) to sell the stock to maximize your profit. Return the maximum profit that you can get from a given input. If you can't profit, return 0.

max_profit.ml

open Base
open Stdio

let max_profit (indices : int list) =
  let rec calculate_profit profit = function
    | [] -> profit
    | buy :: rest ->
      let sell = List.fold rest ~init:buy ~f:max in
      calculate_profit (max (sell - buy) profit) rest
  in
  calculate_profit 0 indices
;;

let%expect_test "max profit" =
  let indices = [ 7; 1; 5; 3; 6; 4 ] in
  let profit = max_profit indices in
  printf "%d\n" profit;
  [%expect {| 5 |}]
;;