121. Best Time to Buy and Sell Stock

Say you have an array for which the ith element is the price of a given stock on day i.

If you were only permitted to complete at most one transaction (i.e., buy one and sell one share of the stock), design an algorithm to find the maximum profit.

Note that you cannot sell a stock before you buy one.

Example 1:

Input: [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.
             Not 7-1 = 6, as selling price needs to be larger than buying price.

Example 2:

Input: [7,6,4,3,1]
Output: 0
Explanation: In this case, no transaction is done, i.e. max profit = 0.

# @param {Integer[]} prices
# @return {Integer}
def max_profit(prices)
  return 0 if prices.empty?
  
  prof_arry = []
  
  for i in 0...prices.length
    if prices[i] < prices[i...prices.length].max
      prof_arry << prices[i...prices.length].max - prices[i]
    end
    
  end
  
  if prof_arry.empty?
    return 0
  else
    prof_arry.max
  end
  
end