```
public class Solution {
public String addStrings(String num1, String num2) {
if(num1 == null || num2 == null) return null;
StringBuilder sb = new StringBuilder();
int carry = 0;
int l1 = num1.length()-1;
int l2 = num2.length()-1;
while(l1>=0 || l2>=0){
int n1 = l1>=0 ? num1.charAt(l1)-'0' : 0;
int n2 = l2>=0 ? num2.charAt(l2)-'0' : 0;
int sum = n1 + n2 + carry;
carry = sum > 9 ? 1 : 0;
sb.insert(0, sum%10);
l1--;
l2--;
}
if(carry == 1) sb.insert(0, 1);
return sb.toString();
}
}
```