415\. Add Strings

Easy

Given two non-negative integers, `num1` and `num2` represented as string, return _the sum of_ `num1` _and_ `num2` _as a string_.

You must solve the problem without using any built-in library for handling large integers (such as `BigInteger`). You must also not convert the inputs to integers directly.

**Example 1:**

**Input:** num1 = "11", num2 = "123"

**Output:** "134" 

**Example 2:**

**Input:** num1 = "456", num2 = "77"

**Output:** "533" 

**Example 3:**

**Input:** num1 = "0", num2 = "0"

**Output:** "0" 

**Constraints:**

*   <code>1 <= num1.length, num2.length <= 10<sup>4</sup></code>
*   `num1` and `num2` consist of only digits.
*   `num1` and `num2` don't have any leading zeros except for the zero itself.