#415字符串相加

final class Solution {
    public String addStrings(String num1, String num2) {
        StringBuilder reversed = new StringBuilder(Math.max(num1.length(), num2.length()) + 1);
        int first = num1.length() - 1;
        int second = num2.length() - 1;
        int carry = 0;
        while (first >= 0 || second >= 0 || carry != 0) {
            int left = first >= 0 ? num1.charAt(first--) - '0' : 0;
            int right = second >= 0 ? num2.charAt(second--) - '0' : 0;
            int sum = left + right + carry;
            reversed.append(sum % 10);
            carry = sum / 10;
        }
        return reversed.reverse().toString();
    }
}