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.

整体思路是：先处理乘除，遇到加减先压栈

public int calculate2(String s) {
        if(s==null){
            throw  new IllegalArgumentException();
        }

        int d=0;//记录压栈的值
        int res=0;  //记录最后的结果

        Character sign='+';  //记录的是前一个符号，也就是数字前的符号

        Stack nums=new Stack<>();

        for(int i=0;i='0'){
                d=d*10+s.charAt(i)-'0';
            }

            if(s.charAt(i)<'0' && s.charAt(i)!=' '|| i==s.length()-1){
                if(sign=='+') nums.push(d);
                if(sign=='-') nums.push(-d);

                if(sign=='*' || sign=='/'){
                    int temp=sign=='*'?nums.pop()*d:nums.pop()/d;
                    nums.push(temp);
                }
               //压栈后重新记录符号和数字
                sign=s.charAt(i);
                d=0;
            }
        }
     //处理加减
        while (!nums.empty()){
            res+=nums.pop();
        }

        System.out.print(res);
        return res;
    }