Add Two Arbitrarily Long Non-Negative Integers Given as Decimal Strings
Company: Meta
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: medium
Interview Round: Other
You are given two non-negative integers written as strings of decimal digits. Return their sum, also written as a string of decimal digits.
The inputs can be thousands of digits long, far beyond any fixed-width integer type. Do not convert either input as a whole into a numeric type and do not use a big-integer library: the sum has to be produced from the digit strings themselves.
### Function Signature
```python
def add_decimal_strings(num1: str, num2: str) -> str:
```
### Rules
- Each input contains only the characters `0` through `9`.
- An input never has leading zeros, except the single-character string `"0"`.
- The two inputs may have different lengths.
- The returned string must contain only decimal digits and have no leading zeros; a sum of zero is returned as `"0"`.
### Constraints
- `1 <= len(num1) <= 10^4`
- `1 <= len(num2) <= 10^4`
- The represented values go far beyond 2^31 - 1 and 2^53, which is why whole-number conversion is ruled out; the answer is compared as a string, so checking it involves no numeric overflow.
### Examples
**Example 1**
```text
Input: num1 = "11", num2 = "123"
Output: "134"
```
**Example 2**
```text
Input: num1 = "999", num2 = "1"
Output: "1000"
```
The final carry adds a new leading digit, so the result is longer than both inputs.
**Example 3**
```text
Input: num1 = "0", num2 = "0"
Output: "0"
```
Overview: Add two non-negative integers that are given as decimal digit strings, possibly thousands of digits long, and return the sum as a string without converting to built-in numbers. Tests digit-by-digit addition with carries, inputs of unequal length and canonical output without leading zeros.
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