Convert InFix String to PostFix String

gistfile1.cs

 private static string ConvertInFixToPostFix(string inFixExpression)
        {
            if (inFixExpression == null || inFixExpression.Trim().Length == 0)
                return inFixExpression;
            inFixExpression = inFixExpression.Replace("(", " ( ");
            inFixExpression = inFixExpression.Replace(")", " ) ");
            inFixExpression = inFixExpression.ToLower();

            string[] tokens = inFixExpression.Split(new string[] { }, StringSplitOptions.RemoveEmptyEntries);
            if (tokens.Length == 0 || tokens.Length == 1)
                return inFixExpression;

            Queue<string> outputQueue = new Queue<string>(tokens.Length);
            Stack<string> operatorStack = new Stack<string>();
            for (int i = 0; i < tokens.Length; i++)
            {
                switch (tokens[i])
                {
                    //a simple operator
                    case "and":
                    case "or":
                        while (operatorStack.Count > 0 &&
                          (operatorStack.Peek() == "and" || operatorStack.Peek() == "or" || operatorStack.Peek() == "not"))
                            outputQueue.Enqueue(operatorStack.Pop());
                        operatorStack.Push(tokens[i]);
                        break;

                    case "not":
                        operatorStack.Push(tokens[i]);
                        break;

                    case "(":
                        operatorStack.Push(tokens[i]);
                        break;

                    case ")":
                        while (operatorStack.Count > 0 && operatorStack.Peek() != "(")
                            outputQueue.Enqueue(operatorStack.Pop());
                        if (operatorStack.Count == 0)
                            throw new ArgumentException("Parenthesis mismatch in the expression: " + inFixExpression);
                        operatorStack.Pop(); //pop "("
                        break;

                    //a category
                    default:
                        outputQueue.Enqueue(tokens[i]);
                        break;
                }
            }
            while (operatorStack.Count != 0)
                outputQueue.Enqueue(operatorStack.Pop());
            string postFixExpression = string.Empty;
            foreach (string token in outputQueue)
            {
                postFixExpression += token + " ";
            }
            return postFixExpression;
        }