<p>You are given an integer <code>n</code>. There are <code>n</code> rooms numbered from <code>0</code> to <code>n - 1</code>.</p>

<p>You are given a 2D integer array <code>meetings</code> where <code>meetings[i] = [start<sub>i</sub>, end<sub>i</sub>]</code> means that a meeting will be held during the <strong>half-closed</strong> time interval <code>[start<sub>i</sub>, end<sub>i</sub>)</code>. All the values of <code>start<sub>i</sub></code> are <strong>unique</strong>.</p>

<p>Meetings are allocated to rooms in the following manner:</p>

<ol>
	<li>Each meeting will take place in the unused room with the <strong>lowest</strong> number.</li>
	<li>If there are no available rooms, the meeting will be delayed until a room becomes free. The delayed meeting should have the <strong>same</strong> duration as the original meeting.</li>
	<li>When a room becomes unused, meetings that have an earlier original <strong>start</strong> time should be given the room.</li>
</ol>

<p>Return<em> the <strong>number</strong> of the room that held the most meetings. </em>If there are multiple rooms, return<em> the room with the <strong>lowest</strong> number.</em></p>

<p>A <strong>half-closed interval</strong> <code>[a, b)</code> is the interval between <code>a</code> and <code>b</code> <strong>including</strong> <code>a</code> and <strong>not including</strong> <code>b</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> n = 2, meetings = [[0,10],[1,5],[2,7],[3,4]]
<strong>Output:</strong> 0
<strong>Explanation:</strong>
- At time 0, both rooms are not being used. The first meeting starts in room 0.
- At time 1, only room 1 is not being used. The second meeting starts in room 1.
- At time 2, both rooms are being used. The third meeting is delayed.
- At time 3, both rooms are being used. The fourth meeting is delayed.
- At time 5, the meeting in room 1 finishes. The third meeting starts in room 1 for the time period [5,10).
- At time 10, the meetings in both rooms finish. The fourth meeting starts in room 0 for the time period [10,11).
Both rooms 0 and 1 held 2 meetings, so we return 0. 
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 3, meetings = [[1,20],[2,10],[3,5],[4,9],[6,8]]
<strong>Output:</strong> 1
<strong>Explanation:</strong>
- At time 1, all three rooms are not being used. The first meeting starts in room 0.
- At time 2, rooms 1 and 2 are not being used. The second meeting starts in room 1.
- At time 3, only room 2 is not being used. The third meeting starts in room 2.
- At time 4, all three rooms are being used. The fourth meeting is delayed.
- At time 5, the meeting in room 2 finishes. The fourth meeting starts in room 2 for the time period [5,10).
- At time 6, all three rooms are being used. The fifth meeting is delayed.
- At time 10, the meetings in rooms 1 and 2 finish. The fifth meeting starts in room 1 for the time period [10,12).
Room 0 held 1 meeting while rooms 1 and 2 each held 2 meetings, so we return 1. 
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= n &lt;= 100</code></li>
	<li><code>1 &lt;= meetings.length &lt;= 10<sup>5</sup></code></li>
	<li><code>meetings[i].length == 2</code></li>
	<li><code>0 &lt;= start<sub>i</sub> &lt; end<sub>i</sub> &lt;= 5 * 10<sup>5</sup></code></li>
	<li>All the values of <code>start<sub>i</sub></code> are <strong>unique</strong>.</li>
</ul>

class Solution {
public:
    int mostBooked(int n, vector<vector<int>>& meetings) {
        // Set of available rooms, sorted by room index
        set<int> available;
        for (int i = 0; i < n; i++) {
            available.insert(i);
        }

        // Set of busy rooms {end time, room index}, sorted by end time first, then index
        set<pair<long long, int>> busy;

        // Vector to count the number of meetings per room
        vector<int> roomCount(n, 0);

        // Sort meetings by their start time
        sort(meetings.begin(), meetings.end());

        for (const auto& meeting : meetings) {
            long long start = meeting[0];
            long long end = meeting[1];

            // Free up rooms that are now available before this meeting starts
            while (!busy.empty() && busy.begin()->first <= start) {
                available.insert(busy.begin()->second);
                busy.erase(busy.begin());
            }

            if (!available.empty()) {
                // Use the earliest available room by index
                int room = *available.begin();
                available.erase(available.begin());
                busy.insert({end, room});
                roomCount[room]++;
            } else {
                // No available rooms, use the room that will be free the earliest
                auto nextFree = *busy.begin();
                busy.erase(busy.begin());
                int room = nextFree.second;
                long long newEnd = nextFree.first + (end - start);
                busy.insert({newEnd, room});
                roomCount[room]++;
            }
        }

        // Find the room with the most meetings
        int maxRoom = 0;
        for (int i = 1; i < n; i++) {
            if (roomCount[i] > roomCount[maxRoom]) {
                maxRoom = i;
            }
        }

        return maxRoom;
    }
};

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