227. 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.

代码如下：（超时）

 public class Solution {
     public int calculate(String s) {
         s=s.replace(" ","");
         s=s.replace("+"," + ");s=s.replace("-"," - ");s=s.replace("*"," * ");s=s.replace("/"," / ");
         String[] ss=s.split(" ");
         List<Integer> list=new ArrayList<>();
         List<String> lists=new ArrayList<>();

         for(int i=0;i<ss.length;i++)
         {
             if(ss[i].equals("+"))
             lists.add(ss[i]);
             else if(ss[i].equals("-"))
             lists.add(ss[i]);
             else if(ss[i].equals("*"))
             {
                 int a=list.get(list.size()-1);
                 list.remove(list.size()-1);
                 int b=Integer.valueOf(ss[i+1]);
                 i++;
                 list.add(a*b);
             }
             else if(ss[i].equals("/"))
             {
                 int a=list.get(list.size()-1);
                 list.remove(list.size()-1);
                 int b=Integer.valueOf(ss[i+1]);
                 i++;
                 if(a!=0)
                 list.add(a/b);
                 else
                  list.add(0);
             }
             else
              list.add(Integer.valueOf(ss[i]));
         }

         while(lists.size()>0)
         {
             int a=list.get(0);
                 list.remove(0);
             int b=list.get(0);

             String c=lists.get(0);
                 lists.remove(0);
             if(c.equals("+"))
             list.set(0,a+b);
             else
              list.set(0,a-b);
         }

         return list.get(0);
     }
 }