cs: add «731. My Calendar II»

Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/my-calendar-ii.cs

@@ -0,0 +1,31 @@
+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;
+    }
+}