Reschedule Meetings for Maximum Free Time II — C++

// Time:  O(n)
// Space: O(1)

// array
class Solution {
public:
    int maxFreeTime(int eventTime, vector<int>& startTime, vector<int>& endTime) {
        const auto& topk = [&](int k) {  // Time: O(k * n)
            vector<pair<int, int>> topk(k, pair(numeric_limits<int>::min(), numeric_limits<int>::min()));
            for (int i = 0; i < size(startTime); ++i) {
                auto x = pair(startTime[i] - endTime[i], endTime[i]);
                for (auto& y : topk) {
                    if (x > y) {
                        swap(x, y);
                    }
                }
            }
            return topk;
        };

        startTime.emplace_back(eventTime);
        endTime.emplace(begin(endTime), 0);
        const auto& top3 = topk(3);
        int result = 0;
        for (int i = 0, curr = 0; i + 1 < size(startTime); ++i) {
            int l = 0;
            for (const auto& [mx, e] : top3) {
                if (e != endTime[i] && e != endTime[i+1] && endTime[i + 1] - startTime[i] <= mx) {
                    l = endTime[i+1] - startTime[i];
                    break;
                }
            }
            result = max(result, (startTime[i] - endTime[i]) + (startTime[i + 1] - endTime[i + 1]) + l);
        }
        return result;
    }
};