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