Leetcode: Basic Calculator

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

 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 sign = 1;
 6         int num = 0;
 7         int sum = 0;
 8         for (int i=0; i<s.length(); i++) {
 9             char c = s.charAt(i);
10             if (c>=‘0‘ && c<=‘9‘) {
11                 if (i<s.length()-1 && s.charAt(i+1)>=‘0‘ && s.charAt(i+1)<=‘9‘) {
12                     num = num * 10 + (int)(c - ‘0‘);
13                     continue;
14                 }
15                 else num = num * 10 + (int)(c - ‘0‘);
16                 sum += sign * num;
17                 num = 0;
18             }
19             if (c == ‘+‘) sign = 1;
20             if (c == ‘-‘) sign = -1;
21             if (c == ‘(‘) {
22                 stack.push(sum);
23                 stack.push(sign);
24                 sign = 1;
25                 sum = 0;
26             }
27             if (c == ‘)‘) {
28                 sum = stack.pop()*sum + stack.pop();
29                 sign = 1;
30             }
31         }
32         return sum;
33     }
34 }