feat(challenges): add single-number

challenges/single-number/README.md

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+# single-number
+
+Created by [@theoludwig](https://github.com/theoludwig) on 21 August 2023.
+
+## Instructions
+
+Given a **non-empty** array of integers, every element appears twice except for one. Find that single one.
+
+You must implement a solution with a linear runtime complexity and use only constant extra space.
+
+### Constraints
+
+- $$1 <= numbers.length <= 3 * 10^4$$
+- $$-3 * 10^4 <= numbers[index] <= 3 * 10^4$$
+- Each element in the array appears twice except for one element which appears only once.
+
+## Source
+
+[LeetCode - Single Number](https://leetcode.com/problems/single-number/)
+
+## Examples
+
+See the `test` folder for examples of input/output.
+
+### Example 1
+
+#### Input
+
+```txt
+4
+1
+2
+1
+2
+```
+
+#### Output
+
+```txt
+4
+```

challenges/single-number/test/1/input.txt

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+4
+1
+2
+1
+2

challenges/single-number/test/1/output.txt

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+4

challenges/single-number/test/2/input.txt

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+2
+2
+1

challenges/single-number/test/2/output.txt

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+1

challenges/single-number/test/3/input.txt

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+1

challenges/single-number/test/3/output.txt

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+1