LeetCode-Best time to buy and sell stock II

很简单，稍微观察一下就可以得出规律：每次买入之后在最高的时候卖出，

只需用low和high来记录买入、卖出的价格，遇到更高价格的时候更新high，否则卖出，

然后同时更新low和high。结束之后最后一次的收入=high - low不要忘了加进去！

class Solution {
public:
    int maxProfit(vector<int> &prices) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (prices.empty()) {
            return 0;
        }
        int low = prices[0];
        int high = prices[0];
        int sum = 0;
        for (size_t i = 1; i < prices.size(); ++i) {
            if (prices[i] > high) {
                high = prices[i];
            }
            else {
                sum += (high - low);
                low = prices[i];
                high = prices[i];
            }
        }
        sum += (high - low);
        return sum;
    }
};