3652. Best Time to Buy and Sell Stock using Strategy

You are given two integer arrays prices and strategy, where:

• prices[i] is the price of a given stock on the i^th day.
• strategy[i] represents a trading action on the i^th day, where:
  • -1 indicates buying one unit of the stock.
  • 0 indicates holding the stock.
  • 1 indicates selling one unit of the stock.

You are also given an even integer k, and may perform at most one modification to strategy. A modification consists of:

• Selecting exactly k consecutive elements in strategy.
• Set the first k / 2 elements to 0 (hold).
• Set the last k / 2 elements to 1 (sell).

The profit is defined as the sum of strategy[i] * prices[i] across all days.

Return the maximum possible profit you can achieve.

Note: There are no constraints on budget or stock ownership, so all buy and sell operations are feasible regardless of past actions.

class Solution {
public:
    long long maxProfit(vector<int>& prices, vector<int>& strategy, int k) {
        vector<long long> pre{0}, pos{0};
        long long res = LLONG_MIN, n = prices.size();
        for(int i = 0; i < n; i++) {
            pre.push_back(pre.back() + prices[i] * strategy[i]);
            pos.push_back(pos.back() + prices[i]);
        }
        for(int i = 0; i <= n - k; i++) {
            long long now = pre[i] + pre[n] - pre[i+k] + pos[i+k] - pos[i + k / 2];
            res = max(res, now);
        }
        res = max(res, pre.back());
        return res;
    }
};