LeetCode: Basic Calculator

Java:
 1 public class Solution {
 2     public int calculate(String s) {
 3         if (s == null || s.length() == 0) return 0;
 4         Stack<Integer> stack = new Stack<Integer>();
 5         int ans = 0;
 6         int sign = 1;
 7         for (int i = 0; i < s.length(); i++)
 8         {
 9             char c = s.charAt(i);
10             if (Character.isDigit(c))
11             {
12                 int cur = c - '0';
13                 while (i+1 < s.length() && Character.isDigit(s.charAt(i+1)))
14                 {
15                     cur = 10 * cur + s.charAt(++i) - '0';
16                 }
17                 ans += sign * cur;
18             }
19             else if (c == '-')
20             {
21                 sign = -1;
22             }
23             else if (c == '+')
24             {
25                 sign = 1;
26             }
27             else if (c == '(')
28             {
29                 stack.push(ans);
30                 ans = 0;
31                 stack.push(sign);
32                 sign = 1;
33             }
34             else if (c == ')')
35             {
36                 ans = stack.pop() * ans + stack.pop();
37                 sign = 1;
38             }
39         }
40         return ans;
41     }
42 }