Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

You may assume that the given expression is always valid.

Some examples:

"1 + 1" = 2
" 2-1 + 2 " = 3
"(1+(4+5+2)-3)+(6+8)" = 23

Note: Do not use the eval built-in library function.

 1 public class Solution {
 2     public int Calculate(string s) {
 3         var nums = new Stack<int>();
 4         var ops = new Stack<char>();
 5         
 6         var sc = s.ToCharArray();
 7         
 8         for (int
 
 i = 0; i < sc.Length; i++)
 9         {
10             if (sc[i] == ‘ ‘)
11             {
12                 continue;
13             }
14             else if (sc[i] == ‘(‘ || sc[i] == ‘+‘ || sc[i] == ‘-‘ )
15             {
16                 ops.Push(sc[i]);
17             }
18             else if
 
 (sc[i] == ‘)‘)
19             {
20                 // the top should be ‘(‘
21                 ops.Pop();
22                 
23                 if (ops.Count > 0)
24                 {
25                     var op = ops.Pop();
26                     var n2 = nums.Pop();
27                     var n1 = nums.Pop();
28 
29                     nums.Push(Calculate(n1, n2, op));
30                 }
31             }
32             else
33             {
34                 int n1 = 0;
35                 
36                 while (i < sc.Length && (int)sc[i] >= (int)‘0‘ && (int)sc[i] <= (int)‘9‘)
37                 {
38                     n1 = n1 * 10 + (int)sc[i] - (int)‘0‘;
39                     i++;
40                 }
41                 
42                 i--;
43                 
44                 if (ops.Count == 0 || ops.Peek() == ‘(‘)
45                 {
46                     nums.Push(n1);
47                 }
48                 else 
49                 {
50                     nums.Push(Calculate(nums.Pop(), n1, ops.Pop()));
51                 }
52             }
53         }
54         
55         return nums.Pop();
56     }
57     
58     private int Calculate(int n1, int n2, char op)
59     {
60         return op == ‘+‘ ? n1 + n2 : n1 - n2;
61     }
62 }

Leetcode 224: Basic Calculator