public static String add(String num1, String num2) {
        if (num1 == null || num1.isEmpty()) return num2;
        if (num2 == null || num2.isEmpty()) return num1;
        int size = Math.min(num1.length(), num2.length());
        StringBuilder temp1 = new StringBuilder(num1).reverse();
        StringBuilder temp2 = new StringBuilder(num2).reverse();
        StringBuilder result = new StringBuilder();
        int bitUp = 0;
        for (int i = 0; i < size; i++) {
            char char1 = (i >= temp1.length()) ? '0' : temp1.charAt(i);
            char char2 = (i >= temp2.length()) ? '0' : temp2.charAt(i);
            int sum = char1 + char2;
            int value = sum - 2 * '0' + bitUp;
            bitUp = value / 10;
            value = value % 10;
            result.append(value);
        }
        return result.reverse().toString();
    }