> For the complete documentation index, see [llms.txt](https://yyloumike.gitbook.io/leetcode/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://yyloumike.gitbook.io/leetcode/string/0415.-add-strings.md).

# 0415. Add Strings

Easy  |  String + Math  |  28 ms (97.36%),  14.2 MB (83.13%)

{% tabs %}
{% tab title="❓ Problem Statement" %}

> Source: [LeetCode - Add Strings](https://leetcode.com/problems/add-strings/)\
> GitHub: [Solution / Performance](https://github.com/yylou/leetcode/tree/main/0415-add-strings)

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**.
{% endtab %}

{% tab title="✍🏻 Constraints & Example" %}
**Constraints:**

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

```
Input: num1 = "11", num2 = "123"
Output: "134"

Input: num1 = "456", num2 = "77"
Output: "533"

Input: num1 = "0", num2 = "0"
Output: "0"
```

{% endtab %}
{% endtabs %}

{% tabs %}
{% tab title="💡 Ideas" %}
{% hint style="info" %}
**Sum each character reversely** by **index indicators** starting from the end.
{% endhint %}

Since **two strings might have different lengths**, we need to take care of the condition *when either one of strings meets the beginning first*.
{% endtab %}
{% endtabs %}

{% tabs %}
{% tab title=" 🤖 Python3" %}

```python
class Solution:
    def addStrings(self, num1: str, num2: str) -> str:
        
        # ==================================================
        #  String + Math                                   =
        # ==================================================
        # time  : O(max(m,n))
        # space : O(1)
        
        ans, carry = '', 0
        
        p1, p2 = len(num1) - 1, len(num2) - 1
        while(p1 >= 0 or p2 >= 0 or carry > 0):
            val1 = ord(num1[p1]) - ord('0') if p1 >= 0 else 0
            val2 = ord(num2[p2]) - ord('0') if p2 >= 0 else 0
            val = val1 + val2 + carry
            carry = val // 10
            val = val % 10
            
            ans = str(val) + ans
            p1 -= 1
            p2 -= 1
            
        return ans
```

{% endtab %}

{% tab title=" 🤖 Java" %}

```java
class Solution {
    /**
     * @time  : O(max(m,n))
     * @space : O(1)
     */
    
    public String addStrings(String num1, String num2) {
        int p1 = num1.length() - 1, p2 = num2.length() - 1, carry = 0;
        
        StringBuilder ans = new StringBuilder();
        
        while(p1 >= 0 || p2 >= 0 || carry > 0) {
            int val1 = (p1 >= 0) ? num1.charAt(p1--) - '0' : 0;
            int val2 = (p2 >= 0) ? num2.charAt(p2--) - '0' : 0;
            int val = val1 + val2 + carry;
            carry = val / 10;
            val = val % 10;
            
            ans.append(val);
        }
        
        return ans.reverse().toString();
    }
}
```

{% endtab %}
{% endtabs %}