LeetCode： Best Time to Buy and Sell Stock

Question

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.

Example 1:

Input: prices = [7,1,5,3,6,4]

Output: 5

Explanation: Buy on day 2 (price = 1) and sell on day 5 (price = 6), profit = 6-1 = 5. Note that buying on day 2 and selling on day 1 is not allowed because you must buy before you sell.

Example 2:

Input: prices = [7,6,4,3,1]

Output: 0

Explanation: In this case, no transactions are done and the max profit = 0.

Solution

def max_profit(prices)
  min_price = prices.first
  max_profit = 0

  prices.each do |current_price|
    if current_price < min_price
      min_price = current_price
    else
      max_profit = [current_price - min_price, max_profit].max
    end
  end

  return max_profit
end