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）

 1 public class Solution {
 2     public int calculate(String s) {
 3         s=s.replace(" ","");
 4         s=s.replace("+"," + ");s=s.replace("-"," - ");s=s.replace("*"," * ");s=s.replace("/"," / ");
 5         String[] ss=s.split(" ");
 6         List<Integer> list=new ArrayList<>();
 7         List<String> lists=new ArrayList<>();
 8         
 9         for(int i=0;i<ss.length;i++)
10         {
11             if(ss[i].equals("+"))
12             lists.add(ss[i]);
13             else if(ss[i].equals("-"))
14             lists.add(ss[i]);
15             else if(ss[i].equals("*"))
16             {
17                 int a=list.get(list.size()-1);
18                 list.remove(list.size()-1);
19                 int b=Integer.valueOf(ss[i+1]);
20                 i++;
21                 list.add(a*b);
22             }
23             else if(ss[i].equals("/"))
24             {
25                 int a=list.get(list.size()-1);
26                 list.remove(list.size()-1);
27                 int b=Integer.valueOf(ss[i+1]);
28                 i++;
29                 if(a!=0)
30                 list.add(a/b);
31                 else
32                  list.add(0);
33             }
34             else
35              list.add(Integer.valueOf(ss[i]));
36         }
37 
38         while(lists.size()>0)
39         {
40             int a=list.get(0);
41                 list.remove(0);
42             int b=list.get(0);
43 
44             String c=lists.get(0);
45                 lists.remove(0);
46             if(c.equals("+"))
47             list.set(0,a+b);
48             else
49              list.set(0,a-b);
50         }
51         
52         return list.get(0);
53     }
54 }