cs: add «189. Rotate Array»

URL:	https://leetcode.com/problems/rotate-array/
Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/rotate-array.cs

@@ -0,0 +1,15 @@
+public class Solution {
+    public void Rotate(int[] nums, int k) {
+        void Reverse(int begin, int end) {
+            for (/* no-op */; begin < end; ++begin, --end) {
+                (nums[begin], nums[end]) = (nums[end], nums[begin]);
+            }
+        }
+
+        k %= nums.Length;
+
+        Reverse(0, nums.Length - 1);
+        Reverse(0, k - 1);
+        Reverse(k, nums.Length - 1);
+    }
+}