Buy low and sell high

• find the lowest point in the prices
• find the highest point in the prices
• most important: the lowest point must not behind the highest point

so walk through the prices, see check if there is more profit at current point. (profit = current - lowest before current)

foreach p in prices

  lowest = MIN(p, lowest)
  
  current_profit = p - lowest
  
  maxprofit = MAX(maxprofit, current_profit)