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:
1. "3+2*2" = 7
2. " 3/2 " = 1
3. " 3+5 / 2 " = 5

Note : Do not use the eval built-in library function.
Solution:
1. public class Solution {
2. public int calculate(String s) {
3. if (s == null || s.length() == 0)
4. return 0;
5. 6. s = "+" + s + "+";
7. char sign = '+';
8. char[] a = s.toCharArray();
9. int n = s.length(), res = 0, num = 0;
10. Stack stack = new Stack();
11. 12. for (int i = 0; i < n; i++) {
13. if (Character.isDigit(a[i]))
14. num = num * 10 + a[i] - '0';
15. 16. if (isOperator(a[i])) {
17. if (sign == '+') stack.push(num);
18. if (sign == '-') stack.push(-num);
19. if (sign == '') stack.push(stack.pop() * num);
20. if (sign == '/') stack.push(stack.pop() / num);
21. 22. num = 0;
23. 24. sign = a[i];
25. }
26. }
27. 28. for (int i : stack)
29. res += i;
30. 31. return res;
32. }
33. 34. boolean isOperator(char c) {
35. return c == '+' || c == '-' || c == '' || c == '/';
36. }
37. }