227. 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:
“3+2*2” = 7
” 3/2 ” = 1
” 3+5 / 2 ” = 5
Note: Do not use the eval built-in library function.

public class Solution {    public int calculate(String s) {        if (s == null || s.length() == 0) return 0;        s = s.replaceAll(" ", "");        if (s.length() == 0) return 0;        int num = 0;        char sign = '+';        Stack<Integer> stack = new Stack<Integer>();        for (int i = 0; i < s.length(); i++) {            if (Character.isDigit(s.charAt(i))) {                num = num * 10 + s.charAt(i)-'0';            }            if (!Character.isDigit(s.charAt(i)) || i == s.length()-1) {                if (sign == '+') {                    stack.push(num);                }                if (sign == '-') {                    stack.push(-num);                }                if (sign == '*') {                    stack.push(stack.pop()*num);                }                if (sign == '/') {                    stack.push(stack.pop()/num);                }                sign = s.charAt(i);                num = 0;            }        }        int res = 0;        for (int i : stack) res += i;        return res;    }}