分析：

class Solution {
public:
    int maxProfit(vector<int>& prices) {
        int n = prices.size();
        vector<vector<int>> dp(n, vector<int>(3));
        dp[0][0] = -prices[0];
        for (int i = 1; i < n; i++) {
            dp[i][0] = max(dp[i - 1][0], dp[i - 1][1] - prices[i]);
            dp[i][1] = max(dp[i - 1][1], dp[i - 1][2]);
            dp[i][2] = dp[i - 1][0] + prices[i];
        }
        return max(dp[n - 1][1], dp[n - 1][2]);
    }
};

分析：
f[i]表示第i天后，是买入 状态
g[i]表示第i天后，是可买入 状态

class Solution {
public:
    int maxProfit(vector<int>& prices, int fee) {
        int n = prices.size();
        vector<int> f(n), g(n);
        f[0] = -prices[0] - fee;
        for (int i = 1; i < n; i++) {
            f[i] = max(g[i - 1] - prices[i] - fee, f[i - 1]);
            g[i] = max(f[i - 1] + prices[i], g[i - 1]);
        }
        return g[n - 1];
    }
};