买卖股票的最佳时机

       int ret = 0;
        if(prices.size()<2)
            return ret;
        int lowest = prices[0];
        for(int i=1;i<prices.size();i++)
        {
            int cur = prices[i];
            ret = max(ret,cur-lowest);
            lowest  = min(lowest,cur);
        }
        return ret;
    }
};