cs: add «1544. Make The String Great»

Signed-off-by: Matej Focko <mfocko@redhat.com>

cs/make-the-string-great.cs

@@ -0,0 +1,21 @@
+public class Solution {
+    public string MakeGood(string s) {
+        var st = new List<char>();
+
+        foreach (char c in s) {
+            var idx = st.Count - 1;
+
+            if (
+                idx < 0
+                || Char.ToLower(c) != Char.ToLower(st[idx])
+                || Char.IsUpper(c) == Char.IsUpper(st[idx])
+            ) {
+                st.Add(c);
+            } else {
+                st.RemoveAt(idx);
+            }
+        }
+
+        return new string(st.ToArray());
+    }
+}