# LeetCode 415. Add Strings

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

**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:**

* `1 <= num1.length, num2.length <= 104`
* `num1` and `num2` consist of only digits.
* `num1` and `num2` don't have any leading zeros except for the zero itself.

**Follow up:** Could you solve it without using any built-in `BigInteger` library or converting the inputs to integer directly?

## Solution

[English Version in Youtube](https://youtu.be/UUhGJVIXFOQ)

[中文版解答Youtube Link](https://youtu.be/jOBZnOcjBQc)

[中文版解答Bilibili Link](https://www.bilibili.com/video/BV1Z54y1L7Ho/)

```
class Solution {
public:
    string addStrings(string num1, string num2) {
        string result;
        int carry = 0;
        int i1 = num1.length() - 1;
        int i2 = num2.length() - 1;
        while (i1 >= 0 || i2 >= 0 || carry > 0) {
            int v1 = i1 >= 0 ? num1[i1] - '0' : 0;
            int v2 = i2 >= 0 ? num2[i2] - '0' : 0;
            int sum = v1 + v2 + carry;
            result.push_back(sum % 10 + '0');
            carry = sum / 10;
            i1--;
            i2--;
        }
        
        reverse(begin(result), end(result));
        return result;
    }
};
```
