227. Basic Calculator II

class Solution {
    public int calculate(String s) {
        // stack to keep previous numbers
        Stack<Integer> stack = new Stack<>();
        char sign = '+';
        int num = 0;
        for(int i=0; i<s.length(); i++){
            char ch = s.charAt(i);
            
            //if is number
            if(isDigit(ch)){
                num=10*num+(ch-'0');
            }
            
            if(!isDigit(ch) && ch!=' '|| i==s.length()-1){
                switch(sign){
                    case '+':
                        stack.push(num);
                        break;
                    case '-':
                        stack.push(-1*num);
                        break;
                    case '*':
                        stack.push(num*stack.pop());
                        break;
                    case '/':
                        stack.push(stack.pop()/num);
                        break;
                }
                sign = ch;
                num=0;
            }
        }
        
        int sum = 0;
        while(!stack.isEmpty()){
            sum+=stack.pop();
        }
        return sum;
    }
    
    private boolean isDigit(char ch){
        return ch>='0' && ch<= '9';
    }
}