121. 买卖股票的最佳时机

/**
 * @param {number[]} prices
 * @return {number}
 */
var maxProfit = function (prices) {
  const len = prices.length
  if (len < 2) return 0
  let res = 0,
    minPrice = prices[0]
  for (let i = 1; i < len; i++) {
    res = Math.max(res, prices[i] - minPrice)
    minPrice = Math.min(prices[i], minPrice)
  }
  return res
}