cs: add «2028. Find Missing Observations»

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

cs/find-missing-observations.cs

@@ -0,0 +1,22 @@
+public class Solution {
+    public int[] MissingRolls(int[] rolls, int mean, int n) {
+        var sum = rolls.Sum();
+
+        var remainder = (rolls.Length + n) * mean - sum;
+        if (remainder < n || remainder > 6 * n) {
+            // cannot construct such rolls
+            return [];
+        }
+
+        var (roll, unmatched) = (remainder / n, remainder % n);
+
+        var missing = new int[n];
+        Array.Fill(missing, roll);
+
+        for (int i = 0; i < unmatched; ++i) {
+            ++missing[i];
+        }
+
+        return missing;
+    }
+}