cs: add «2843. Count Symmetric Integers»

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

cs/count-symmetric-integers.cs

@@ -0,0 +1,19 @@
+public class Solution {
+    public int CountSymmetricIntegers(int low, int high) {
+        var count = 0;
+
+        for (var x = low; x <= high; ++x) {
+            if (x < 100 && x % 11 == 0) {
+                ++count;
+            } else if (1000 <= x && x < 10000) {
+                var left = x / 1000 + (x % 1000) / 100;
+                var right = (x % 100) / 10 + x % 10;
+                if (left == right) {
+                    ++count;
+                }
+            }
+        }
+
+        return count;
+    }
+}