LeetCode - Evaluate Reverse Polish Notation

Evaluate Reverse Polish Notation

//
public int evalRPN(String[] tokens) {
        if(tokens == null || tokens.length == 0) return 0;
        Stack<String> stack = new Stack<String>();
        for(int i=0; i<tokens.length; i++){
            String s = tokens[i];
            int value1 = 0, value2 = 0, ret = 0;
            switch(s){
                case "+":
                    value1 = Integer.parseInt(stack.pop());
                    value2 = Integer.parseInt(stack.pop());
                    ret = value1 + value2;
                    stack.push(Integer.toString(ret));
                    break;
                case "-":
                    value1 = Integer.parseInt(stack.pop());
                    value2 = Integer.parseInt(stack.pop());
                    ret = value2 - value1;
                    stack.push(Integer.toString(ret));
                    break;
                case "*":
                    value1 = Integer.parseInt(stack.pop());
                    value2 = Integer.parseInt(stack.pop());
                    ret = value1 * value2;
                    stack.push(Integer.toString(ret));
                    break;
                case "/":
                    value1 = Integer.parseInt(stack.pop());
                    value2 = Integer.parseInt(stack.pop());
                    ret = value2 / value1;
                    stack.push(Integer.toString(ret));
                    break;
                default:
                    stack.push(s);
            }
        }
        if(!stack.isEmpty()){
            return Integer.parseInt(stack.pop());
        } else {
            return 0;
        }
    }
    
//
public int evalRPN(String[] tokens) {
        if(tokens==null || tokens.length==0) return -1;
        Stack<Integer> stack = new Stack<Integer>();
        int ret = -1;
        for(int i=0; i<tokens.length; i++){
            String str = tokens[i];
            if(str.equals("+")){
                ret = stack.pop() + stack.pop();
                stack.push(ret);
            } else if(str.equals("-")){
                int int1 = stack.pop();
                int int2 = stack.pop();
                ret = int2 - int1;
                stack.push(ret);
            }else if(str.equals("*")){
                ret = stack.pop()*stack.pop();
                stack.push(ret);
            }else if(str.equals("/")){
                int int1 = stack.pop();
                int int2 = stack.pop();
                ret = int2/int1;
                stack.push(ret);
            }else{
                stack.push(Integer.parseInt(str));
            }
        }
        return stack.pop();
    }
//
public int evalRPN(String[] tokens) {
        int returnValue = 0;
        String operators = "+-*/";
        Stack<String> stack = new Stack<String>();
        for(String t:tokens){
            if(!operators.contains(t))
                stack.push(t);
            else{
                int a = Integer.valueOf(stack.pop());
                int b = Integer.valueOf(stack.pop());
                int index = operators.indexOf(t);
                switch(index){
                    case 0:
                        stack.push(String.valueOf(a+b));
                        break;
                    case 1:
                        stack.push(String.valueOf(b-a));
                        break;
                    case 2:
                        stack.push(String.valueOf(a*b));
                        break;
                    case 3:
                        stack.push(String.valueOf(b/a));
                        break;
                }
            }
        }
        returnValue = Integer.valueOf(stack.pop());
        return returnValue;
    }