Basic Calculator

Implement a basic calculator to evaluate a simple expression string.
The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

You may assume that the given expression is always valid.

Some examples:

"1 + 1" = 2
" 2-1 + 2 " = 3
"(1+(4+5+2)-3)+(6+8)" = 23

Note: Do not use the eval built-in library function.

题目中只有+ - ( )。遍历字符串，对于每个字符c：

• 如果是数字，则一直遍历到非数字字符，把数字找出，并与结果相加
• 如果是+-符号，将sign设置成对应的值
• 如果是(，将rt和sign压入栈中，重置rt和sign
• 如果是)，将sign和rt弹出栈，并计算结果

    /**
     * 计算带括号的string的值
     * @param s
     * @return
     */
    public int calculate(String s) {
        if(s==null){
            throw  new IllegalArgumentException();
        }

        int res=0; //结果值
        int flag=1; //记录前面的符号，加为1，减为-1

        Stack stack=new Stack<>(); 

        for (int i=0;i