class Solution {

     public int calculate(String s) {

         int result=0,len=s.length(),num=0;

         char op='+';  //初始上一个运算符为加法 上个数字为0

         Stack<Integer> stack=new Stack<Integer>();

         for(int i=0;i<len;i++){

             char c=s.charAt(i);

             if(c>='0'){

                 num=num*10+s.charAt(i)-'0';

             }

             if(c<'0'&&c!=' '||i==len-1){

                 if(op=='+') stack.push(num);

                 if(op=='-') stack.push(-num);

                 if(op=='*'||op=='/'){

                     int temp=(op=='*')?stack.pop()*num:stack.pop()/num;

                     stack.push(temp);

                 }

                 op=s.charAt(i);

                 num=0;

             }

         }

         while(!stack.isEmpty()){

             result+=stack.pop();

         }

         return result;

     }

 }