convert to reverse polish notation.java

//input: expression string, there may be multiple spaces
//return: reverse polish notation string with one space between each element
class Solution {
    public String convertToReversePolish(String exp) {
    	if (exp == null)
    		return null;
    	String res = "";
    	int len = exp.length();
    	Stack<Character> operator = new Stack<Character>(); 
    	Stack<String> reversePolish = new Stack<String>();
    	//avoid checking empty
    	operator.push('#');
    	for (int i = 0; i < len;) {
    		//deal with space
    		while (i < len && exp.charAt(i) == ' ')
    			i++;
    		if (i == len)
    			break;
    		//if is number
    		if (isNum(exp.charAt(i))) {
    			String num = "";
    			while (i < len && isNum(exp.charAt(i))) 
        			num += exp.charAt(i++);
    			reversePolish.push(num);
    		//is operator
    		} else if (isOperator(exp.charAt(i))) {
    			char op = exp.charAt(i);
    			switch (op) {
    			case '(':
    				operator.push(op);
    				break;
    			case ')':
    				while (operator.peek() != '(')
    					reversePolish.push(Character.toString(operator.pop()));
    				operator.pop();
    				break;
				case '+':
				case '-':
					if (operator.peek() == '(')
						operator.push(op);
					else {
						while (operator.peek() != '#' && operator.peek() != '(')
							reversePolish.push(Character.toString(operator.pop()));
						operator.push(op);
					}
					break;
				case '*':
				case '/':
					if (operator.peek() == '(')
						operator.push(op);
					else {
						while (operator.peek() != '#' && operator.peek() != '+' &&
								operator.peek() != '-' && operator.peek() != '(')
							reversePolish.push(Character.toString(operator.pop()));
						operator.push(op);
					}
					break;
				}
    			i++;
    		}
    	}
    	while (operator.peek() != '#')
    		reversePolish.push(Character.toString(operator.pop()));
    	while (!reversePolish.isEmpty())
    		res = res.length() == 0? reversePolish.pop() + res: reversePolish.pop() + " " + res;
    	return res;
    }
    
    public boolean isOperator(char c) {
    	return c == '+' || c == '-' || c == '*' || c == '/' || c == '(' || c == ')';
    }
    
    public boolean isNum(char c) {
    	return c - '0' >= 0 && c - '0' <= 9;
    }
}