121. 买卖股票的最佳时机

思路：每一步都记录最大的利润(当前价格减去之前的最低值)

        int ret = 0;
        int lowest = prices[0];
        int length = prices.size();
        if (0 == length) return 0;
        for (int i = 0; i < length; i ++)
        {
            int currentPrice = prices[i];
            ret = std::max(ret , currentPrice - lowest);
            lowest = std::min(currentPrice, lowest);
        }
        return ret;