max_profit.js

const maxProfit = arr => {
  let bestBuyPrice = 1000000
  let maxProfit = -1

  for(let i = 0; i < arr.length - 1; i++) {
    const currentBuyPrice = arr[i]
    const nextSellPrice = arr[i+1]

    if (currentBuyPrice < bestBuyPrice) bestBuyPrice = currentBuyPrice 

    const currentProfit = nextSellPrice - bestBuyPrice

    if (currentProfit > maxProfit) maxProfit = currentProfit
  }

  return maxProfit
}

const prices1 = [32, 46, 26, 38, 40, 48, 42] // 22
const prices2 = [10, 18, 4, 5, 9, 6, 16, 12] // 12
const prices3 = [10, 9, 8, 8, 7, 6, 5, 4] // 0
const prices4 = [10, 9, 8, 7, 6, 5, 4, 3] // -1

const res1 = maxProfit(prices1)
console.log(res1) // 22
const res2 = maxProfit(prices2)
console.log(res2) // 12
const res3 = maxProfit(prices3)
console.log(res3) // 0
const res4 = maxProfit(prices4)
console.log(res4) // -1