Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.

Note:

1. The length of both num1 and num2 is < 5100.
2. Both num1 and num2 contains only digits 0-9.
3. Both num1 and num2 does not contain any leading zero.
4. You must not use any built-in BigInteger library or convert the inputs to integer directly.

Solution:

```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) {
int a = num1.charAt(i) - '0';
int b = num2.charAt(j) - '0';
int val = (a + b + carry) % 10;
carry = (a + b + carry) / 10;
sb.append(val + "");
i --;
j --;
}
while (i >= 0) {
int a = num1.charAt(i) - '0';
int val = (a + carry) % 10;
carry = (a + carry) / 10;
sb.append(val + "");
i --;
}
while (j >= 0) {
int a = num2.charAt(j) - '0';
int val = (a + carry) % 10;
carry = (a + carry) / 10;
sb.append(val + "");
j --;
}
if (carry > 0) {
sb.append("1");
}
return sb.reverse().toString();
}
}```