若P[k]>=P[k-1]那么在k-1时选择不卖出，否则卖出

class Solution {
public:
    int maxProfit(vector<int>& prices) {
        prices.push_back(0);
        int m=prices[0],ans=0;
        for(int k=1;k<prices.size();++k)
            if(prices[k]<prices[k-1])
                ans+=(prices[k-1]-m),m=prices[k];
        return ans;
    }
};