Leetcode: 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.

Code (Java):

public class Solution {
    public int calculate(String s) {
        if (s == null || s.length() == 0) {
            return 0;
        }
        
        String delim = "[ ]+";
        s = s.replaceAll(delim, "");
        
        Stack<Integer> numStack = new Stack<Integer>();
        Stack<Character> opStack = new Stack<Character>();
        
        int result = 0;
        
        int i = 0;
        while (i < s.length()) {
            char token = s.charAt(i);
            if (isNumber(token)) {
                StringBuffer sb = new StringBuffer();
                while (i < s.length() && isNumber(s.charAt(i))) {
                    sb.append(s.charAt(i));
                    i++;
                }
                
                int val = Integer.valueOf(sb.toString());
                numStack.push(val);
            } else {
                if (opStack.isEmpty() || !hasHigherPrecedence(opStack.peek(), token)) {
                    opStack.push(token);
                    i++;
                } else {
                   calculate(numStack, opStack);
                }
            }
        }
        
        while(!opStack.isEmpty()) {
            calculate(numStack, opStack);
        }
        
        return numStack.pop();
    }
    
    private boolean isNumber(char character) {
        return character >= '0' && character <= '9';
    }
    
    private void calculate(Stack<Integer> numStack, Stack<Character> opStack) {
        int num2 = numStack.pop();
        int num1 = numStack.pop();
        char oprator = opStack.pop();
        
        int result = 0;
        
        switch(oprator) {
            case '+' : result = num1 + num2;
            break;
            
            case '-' : result = num1 - num2;
            break;
            
            case '*' : result = num1 * num2;
            break;
            
            case '/' : result = num1 / num2;
            break;
        }
        
        numStack.push(result);
    }
    
    private boolean hasHigherPrecedence(char str1, char str2) {
        if (str1 == '*'|| str1 == '/') {
            return true;
        }
        
        if ((str1 == '+' || str1 == '-') && (str2 == '+' || str2 == '-')) {
            return true;
        }
        
        return false;
    }
}