class Solution { public String addStrings(String num1, String num2) { StringBuilder sb = new StringBuilder(); int i = num1.length() - 1; int j = num2.length() - 1; int carry = 0; while (i >= 0 || j >= 0 || carry > 0){ if (i >= 0) carry += num1.charAt(i--) - '0'; if (j >= 0) carry += num2.charAt(j--) - '0'; sb.append(carry % 10); carry /= 10; } return sb.reverse().toString(); } }