Help please.

public static void task1(){

        //Creating a scanner for user input.
        Scanner sc = new Scanner(System.in);
        //Asking the user for a infix string
        System.out.println("Hi, please enter a infix string! (This program will convert it into RPN).");
        //Creating a variable for the infix string and also reading the value.
        String infix = sc.nextLine();
        //Creating a variable for the output.
        String output = "", tmp = "";
        //Getting the amount of characters.
        int length = infix.length();


        //Converting the infix string into a RPN string.
        for (int i = 0; i < length; i++){

            char character = infix.charAt(i);
            if (character >= '0' && character <= '9'){
                output = output + character;
            }
            else if (character == '('){
                tmp = character + tmp;
                //tmp = tmp + character;
            }
            else if (character == ')'){

                int j = tmp.indexOf('(');
                if (tmp.contains("(")){
                    output += tmp.substring(0, j);
                    String remove = tmp.substring(0, j+1);
                    tmp = tmp.replace(remove,"");
                    System.out.println(tmp);
                }
                //5+5*5*(2+10)
            }
            else if (character == '+' || character == '-' || character == '*' || character == '/'){
                //tmp = tmp + character;


                char t = tmp.length() > 0 ? tmp.charAt(0): ' ';
                while (tmp.length() > 0 && !(operatorCheck(t) > operatorCheck(character)) && t != '('){
                    output += t;
                    System.out.println("before delete: " + tmp);
                    tmp = tmp.substring(0, tmp.length()-1);
                    System.out.println("delete: " + tmp);
                }
                tmp = character + tmp;
            }

            // test - System.out.println(infix.charAt(i));
            System.out.println("tmp:" + tmp);
            System.out.println("output" + output + "\n");

        }

        output += tmp;
        tmp = "";



        //test print out.
        System.out.println("infix is: " + infix + "\n");
        System.out.println("infix length is: " + length + " chars.\n");
        System.out.println("output: " + output + "\n");
        System.out.println("in tmp: " + tmp + "\n");

    }
    public static int operatorCheck(char operator){
        if (operator == '+' || operator == '-'){
            return 0;
        }
        return 1;
    }