121. 买卖股票的最佳时机

一开始这样子没有用动规

class Solution {
public:
    int maxProfit(vector<int>& prices) {
        int res = 0;
        int cmin = INT_MAX;
        for (int i = 0; i < prices.size(); i ++ ) {
            cmin = min(cmin, prices[i]);
            if (prices[i] > cmin) {
                res = max(res, prices[i] - cmin);
            }
        }
        return res;
    }
};