121. Best Time to Buy and Sell Stock

提议就是又一次买入和卖出，每天价格都不一样，就最大利润，就一次dp，维护最小值就

class Solution {
public:
    int maxProfit(vector<int>& prices) {
        if(prices.size() == 0) return 0;
        int minn, sum;
        minn = prices[0];
        sum = 0;
        for(int i = 1; i < prices.size(); ++ i){
            if(prices[i] < minn){
                minn = prices[i];
                continue;
            }
            else{
                if(prices[i] - minn > sum)
                    sum = prices[i] - minn;
            }
        }
        return sum;
    }