227. Basic Calculator II

class Solution {
    public int calculate(String s) {
        Deque<Integer> operands = new ArrayDeque<>();
        Deque<Character> operators = new ArrayDeque<>();
        
        StringBuilder number = new StringBuilder();
        
        int i = 0;
        while (i < s.length()) {
            char c = s.charAt(i);
            if (c == ' ') {
                i++;
                continue;
            }
            
            // eg. 1 - 2 * 3 / 2
            if (Character.isDigit(c)) {
                number.append(c);
            } else if (c == '+' || c == '-' || c == '*' || c == '/') {
                if (number.length() != 0) {
                    operands.offerFirst(Integer.parseInt(number.toString()));
                    number = new StringBuilder();
                }
                
                if (operators.isEmpty()) {
                    operators.offerFirst(c);
                } else if ((c == '*' || c == '/') && (operators.peek() == '+' || operators.peek() == '-')) {
                    operators.offerFirst(c);
                } else if ((c == '+' || c == '-') && (operators.peek() == '*' || operators.peek() == '/')) {
                    // calculate all previous expressions 
                    while (!operators.isEmpty()) {
                        operands.offerFirst(calculateValue(operands, operators.pollFirst()));
                    }
                    operators.offerFirst(c);
                } else {
                    // only calculate one step
                    operands.offerFirst(calculateValue(operands, operators.pollFirst()));
                    operators.offerFirst(c);
                }
            }
            i++;
        }
        
        if (number.length() != 0) {
            operands.push(Integer.parseInt(number.toString()));
        }
        
        while (!operators.isEmpty()) {
            operands.push(calculateValue(operands, operators.pollFirst()));
        }
        
        return operands.pollFirst();
    }
    
    private int calculateValue(Deque<Integer> operands, char op) {
        int o2 = operands.pollFirst();
        int o1 = operands.pollFirst();
        
        if (op == '+') {
            return o1 + o2;
        } else if (op == '-') {
            return o1 - o2;
        } else if (op == '*') {
            return o1 * o2;
        } else {
            return o1 / o2;
        }
    }
}