Basic Calculator II.java

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