Basic Calculator II

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

You may assume that the given expression is always valid.

Some examples:
"3+2*2" = 7
" 3/2 " = 1
" 3+5 / 2 " = 5
Note: Do not use the eval built-in library function.

给出字符串，然后算出结果，我自己的思路是用Stack来做，如果遇见乘法和除法这种优先级高的算法，那么弹出stack元素，并取得队列的下一个元素，并计算，最后Stack里面只剩下数字和+/-法则，循环计算就可以得到结果

    public int calculate(String s) {
        Set<String> nset = new HashSet<String>();
        for(char i = '0'; i <= '9'; i++){
            nset.add(i + "");
        }
        Set<String> symbol1 = new HashSet<String>();
        symbol1.add("+");symbol1.add("-");
        Set<String> symbol2 = new HashSet<String>();
        symbol2.add("*");symbol2.add("/");
        Stack<String> stack = new Stack<String>();

        StringBuilder sb = new StringBuilder();
        List<String> list = new ArrayList<String>();
        for(int i = 0; i < s.length(); i++){
            if(nset.contains(s.charAt(i) + "")) {
                sb.append(s.charAt(i));
            }else if(s.charAt(i) == ' '){
                continue;
            }else{
                list.add(sb.toString());
                list.add(s.charAt(i) + "");
                sb = new StringBuilder();
            }
        }

        if(!"".equals(sb.toString())){
            list.add(sb.toString());
        }

        for(int i = 0; i < list.size(); i++){
            if(symbol2.contains(list.get(i))){
                int a = Integer.valueOf(stack.pop());
                int b = Integer.valueOf(list.get(i + 1));
                if(list.get(i).equals("*")){
                    stack.push(a * b + "");
                }else{
                    stack.push(a / b + "");
                }
                i ++;
            }else{
                stack.push(list.get(i));
            }
        }
        int result = 0;

        for(int i = 0; i< stack.size(); i++){
            String a = stack.elementAt(i);
            if(symbol1.contains(a)){
                int b = Integer.valueOf(stack.elementAt(i + 1));
                if(a.equals("-")){
                    b = 0 - b;
                }
                result += b;
                i++;
            }else{
                result += Integer.valueOf(a);
            }
        }
        return result;
    }