Add "Best Time to Buy and Sell Stock"

lib/dynamic_programming/best_time_to_buy_and_sell_stock.ex

@@ -0,0 +1,33 @@
+defmodule Algorithms.DynamicProgramming.BestTimeToBuyStock do
+  @moduledoc """
+  Best Time To Buy and Sell Stock 
+  """
+
+  @doc """
+  You are given an array prices where prices[i] is the price of a given stock on the ith day.
+  You want to maximize your profit by choosing a single day to buy one stock and choosing a different day in the future to sell that stock.
+  Return the maximum profit you can achieve from this transaction. If you cannot achieve any profit, return 0.
+
+  https://leetcode.com/problems/best-time-to-buy-and-sell-stock/description/    
+  """
+  @spec max_profit(prices :: [integer]) :: integer
+  def max_profit(prices) do
+    [first_price | remaining] = prices
+
+    {_, stack} =
+      Enum.reduce(remaining, {first_price, []}, fn current, {base, stack} ->
+        res = current - base
+
+        if res > 0 do
+          {base, [res | stack]}
+        else
+          {current, stack}
+        end
+      end)
+
+    case stack do
+      [] -> 0
+      _ -> Enum.max(stack)
+    end
+  end
+end

test/dynamic_programming/best_time_to_buy_and_sell_stock_test.exs

@@ -0,0 +1,15 @@
+defmodule Algorithms.DynamicProgramming.BestTimeToBuyStockTest do
+  alias Algorithms.DynamicProgramming.BestTimeToBuyStock
+
+  use ExUnit.Case
+
+  describe "max_profit/1 - example test cases" do
+    test "max_profit 1" do
+      assert BestTimeToBuyStock.max_profit([7, 1, 5, 3, 6, 4]) == 5
+    end
+
+    test "max_profit 2" do
+      assert BestTimeToBuyStock.max_profit([7, 6, 4, 3, 1]) == 0
+    end
+  end
+end