解法

注意转化子问题的时机，可以一时间一块弹出两个元素。

代码

import java.util.Stack;
class Solution {
    public static int calculate(String s) {
        Stack<String> stack=new Stack<>();
        char[] chars = s.toCharArray();
        int curnum=0;
        boolean hasNum=false;
        for(int i=0;i<chars.length;i++)
        {
            if(chars[i]>='0'&&chars[i]<='9')
            {
                curnum=curnum*10+chars[i]-'0';
                hasNum=true;
            }else if(chars[i]!=' ')
            {
                if(hasNum)
                {
                    if(!stack.isEmpty()&&(stack.peek().equals("*")||stack.peek().equals("/")))
                    {
                        String point=stack.pop();
                        int tempnum=Integer.parseInt(stack.pop());
                        if(point.equals("*"))
                        {
                            stack.push(String.valueOf(tempnum*curnum));
                        }else
                        {
                            stack.push(String.valueOf(tempnum/curnum));
                        }
                    }else
                    {
                        stack.push(String.valueOf(curnum));
                    }
                    curnum=0;
                    hasNum=false;
                }
                stack.push(String.valueOf(chars[i]));
            }
        }
        if(hasNum)
        {
            if(!stack.isEmpty()&&(stack.peek().equals("*")||stack.peek().equals("/")))
            {
                String point=stack.pop();
                int tempnum=Integer.parseInt(stack.pop());
                if(point.equals("*"))
                {
                    stack.push(String.valueOf(tempnum*curnum));
                }else
                {
                    stack.push(String.valueOf(tempnum/curnum));
                }
            }else
            {
                stack.push(String.valueOf(curnum));
            }
        }
        int res=0;
        boolean flag=true;
        for(String str:stack)
        {
             if(str.equals("+"))
            {
                flag=true;
            }else if(str.equals("-"))
            {
                flag=false;
            }else
            {
               if(flag) res+=Integer.parseInt(str);
               else res-=Integer.parseInt(str);
            }
        }
        return res;
    }
}