first time in my life i found a use case for HashSet

Evil Calculator/EquationManager.cs

@@ -2,6 +2,7 @@
 {
     class EquationManager
     {
+        private static readonly HashSet<string> _operators = new() { "+", "-", "*", "/", "^", "√" };
         private ReverseNotationSolver reverseNotationSolver = new();
 
         List<Token> equation = new();
@@ -177,7 +178,7 @@
         private bool IsOperator()
         {
             if (equation.Count == 0) return false;
-            return new[] { "+", "-", "*", "/", "^", "√" }.Contains(equation.Last().value);
+            return _operators.Contains(equation.Last().value);
         }
 
         private bool IsOpen()