cs: add «2300. Successful Pairs of Spells and Potions»

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

cs/successful-pairs-of-spells-and-potions.cs

@@ -0,0 +1,25 @@
+public class Solution {
+    public int[] SuccessfulPairs(int[] spells, int[] potions, long success) {
+        int getSuccessful(long strength) {
+            var (left, right) = (0, potions.Length);
+
+            while (left < right) {
+                var mid = (left + right) / 2;
+                if (strength * potions[mid] >= success) {
+                    right = mid;
+                } else {
+                    left = mid + 1;
+                }
+            }
+
+            return potions.Length - left;
+        }
+
+        // Ensort potions to allow bisection.
+        Array.Sort(potions);
+
+        return spells
+            .Select(s => getSuccessful(s))
+            .ToArray();
+    }
+}