int maxProfit(const vector<int>& prices) {
        if (prices.empty()) return 0;
        auto memo = vector<int>(prices.size(), -1);
        memo.back() = 0;

        for (int i = prices.size() - 2; i >= 0; --i) {
            int res = memo[i + 1]; // do not sell (and do not buy)
            for (int j = i + 1; j < prices.size(); ++j) { // j is day to buy
                auto suffix_res = j + 2 < memo.size() ? memo[j + 2] : 0;
                res = max(res, prices[j] - prices[i] + suffix_res); // sell
            }
            memo[i] = res;
        }

        return memo[0];
    }