LeetCode/cs/my-calendar-ii.cs

public class MyCalendarTwo {
    private SortedDictionary<int, int> counters = [];
    private int maxOverlaps = 2;

    public MyCalendarTwo() { }

    public bool Book(int start, int end) {
        counters[start] = counters.GetValueOrDefault(start, 0) + 1;
        counters[end] = counters.GetValueOrDefault(end, 0) - 1;

        var overlaps = 0;
        foreach (var pair in counters) {
            var (_, count) = (pair.Key, pair.Value);

            overlaps += count;

            if (overlaps > maxOverlaps) {
                counters[start] = counters[start] - 1;
                counters[end] = counters[end] + 1;

                if (counters[start] == 0) {
                    counters.Remove(start);
                }

                return false;
            }
        }

        return true;
    }
}