0415. Add Strings

https://leetcode.com/problems/add-strings

Description

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:

• 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.

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