diff --git a/challenges/ascii-art/README.md b/challenges/ascii-art/README.md
index a075ee1..02ff16f 100644
--- a/challenges/ascii-art/README.md
+++ b/challenges/ascii-art/README.md
@@ -43,13 +43,13 @@ The characters that are not in the intervals `[a-z]` or `[A-Z]` will be shown as
 
 ### Constraints
 
-- 0 < W < 30
-- 0 < H < 30
-- 0 < N < 200
+- $$0 < W < 30$$
+- $$0 < H < 30$$
+- $$0 < N < 200$$
 
 ## Source
 
-- [CodinGame](https://www.codingame.com/training/easy/ascii-art)
+[CodinGame](https://www.codingame.com/training/easy/ascii-art)
 
 ## Examples
 
diff --git a/challenges/caesar-cipher/README.md b/challenges/caesar-cipher/README.md
index e11498d..952fdf0 100644
--- a/challenges/caesar-cipher/README.md
+++ b/challenges/caesar-cipher/README.md
@@ -27,7 +27,7 @@ Create a function that will return the sentence after shifting the alphabet.
 
 ## Source
 
-- [Wikipedia - Caesar cipher](https://en.wikipedia.org/wiki/Caesar_cipher)
+[Wikipedia - Caesar cipher](https://en.wikipedia.org/wiki/Caesar_cipher)
 
 ## Examples
 
diff --git a/challenges/cakes-swerc-2020-2021/README.md b/challenges/cakes-swerc-2020-2021/README.md
index 1a1dc5f..69df1da 100644
--- a/challenges/cakes-swerc-2020-2021/README.md
+++ b/challenges/cakes-swerc-2020-2021/README.md
@@ -32,7 +32,7 @@ available ingredients.
 
 ## Source
 
-- [SWERC 2020–2021 - Problem E: Cake](https://swerc.eu/2020/problems/)
+[SWERC 2020–2021 - Problem E: Cake](https://swerc.eu/2020/problems/)
 
 ## Examples
 
diff --git a/challenges/defibrillators/README.md b/challenges/defibrillators/README.md
index fc8144c..3890e8e 100644
--- a/challenges/defibrillators/README.md
+++ b/challenges/defibrillators/README.md
@@ -48,11 +48,11 @@ The name of the defibrillator located the closest to the user’s position.
 
 ### Constraints
 
-- 0 < `N` < 10 000
+- $$0 < N < 10 000$$
 
 ## Source
 
-- [CodinGame](https://www.codingame.com/training/easy/defibrillators)
+[CodinGame](https://www.codingame.com/training/easy/defibrillators)
 
 ## Examples
 
diff --git a/challenges/heap-algorithm/README.md b/challenges/heap-algorithm/README.md
index d42c990..08207bf 100644
--- a/challenges/heap-algorithm/README.md
+++ b/challenges/heap-algorithm/README.md
@@ -10,7 +10,7 @@ The order of the generated permutations is important, see the example below.
 
 ## Source
 
-- [Heap's Algorithm - Wikipedia](https://en.wikipedia.org/wiki/Heap%27s_algorithm)
+[Heap's Algorithm - Wikipedia](https://en.wikipedia.org/wiki/Heap%27s_algorithm)
 
 ## Examples
 
diff --git a/challenges/look-and-say-sequence-conway/README.md b/challenges/look-and-say-sequence-conway/README.md
index 70aca8e..3368b7d 100644
--- a/challenges/look-and-say-sequence-conway/README.md
+++ b/challenges/look-and-say-sequence-conway/README.md
@@ -20,7 +20,7 @@ Write a program that prints the next term of the **look-and-say sequence**.
 
 ## Source
 
-- [Look-and-say sequence - Wikipedia](https://en.wikipedia.org/wiki/Look-and-say_sequence)
+[Look-and-say sequence - Wikipedia](https://en.wikipedia.org/wiki/Look-and-say_sequence)
 
 ## Examples
 
diff --git a/challenges/offset-arrays/README.md b/challenges/offset-arrays/README.md
index 1484ef5..57b1b4c 100644
--- a/challenges/offset-arrays/README.md
+++ b/challenges/offset-arrays/README.md
@@ -24,15 +24,15 @@ You are given a list of `n` array definitions and your job is to figure out what
 
 ### Constraints
 
-- 1 <= `n` <= 100
-- Array names consist only of uppercase letters A to Z
-- Array lengths are between 1 and 100 (no empty arrays)
-- Indexing operations have at most 50 levels of nesting
-- Indices are always within bounds in the test cases
+- $$1 <= n <= 100$$
+- Array names consist only of uppercase letters A to Z.
+- Array lengths are between 1 and 100 (no empty arrays).
+- Indexing operations have at most 50 levels of nesting.
+- Indices are always within bounds in the test cases.
 
 ## Source
 
-- [CodinGame](https://www.codingame.com/ide/puzzle/offset-arrays)
+[CodinGame](https://www.codingame.com/ide/puzzle/offset-arrays)
 
 ## Examples
 
diff --git a/challenges/prefix-suffix/README.md b/challenges/prefix-suffix/README.md
index 7b90efe..0d39616 100644
--- a/challenges/prefix-suffix/README.md
+++ b/challenges/prefix-suffix/README.md
@@ -20,7 +20,7 @@ Write a programs that takes 2 strings ("words") and prints if one is a prefix/su
 - **Line 1:** `true` if the second word is a **prefix** of the first, `false` otherwise
 - **Line 1:** `true` if the second word is a **suffix** of the first, `false` otherwise
 
-## Source
+## Sources
 
 - [Wikipedia - Prefix](https://en.wikipedia.org/wiki/Prefix)
 - [Wikipedia - Suffix](https://en.wikipedia.org/wiki/Suffix)
diff --git a/challenges/reverse-polish-notation/README.md b/challenges/reverse-polish-notation/README.md
index ac58333..5699789 100644
--- a/challenges/reverse-polish-notation/README.md
+++ b/challenges/reverse-polish-notation/README.md
@@ -20,7 +20,7 @@ All the numbers are integers; you don't need to worry about floating point numbe
 
 ## Source
 
-- [Reverse polish notation - Codewars](https://www.codewars.com/kata/52f78966747862fc9a0009ae)
+[Reverse polish notation - Codewars](https://www.codewars.com/kata/52f78966747862fc9a0009ae)
 
 ## Examples
 
diff --git a/challenges/roman-numerals/README.md b/challenges/roman-numerals/README.md
index 71e4d5d..53132f5 100644
--- a/challenges/roman-numerals/README.md
+++ b/challenges/roman-numerals/README.md
@@ -35,7 +35,7 @@ Here are the rules for building a Roman numeral:
 - **Line 1:** The string : `arabic to roman` or `roman to arabic` to determine how to convert the number
 - **Line 2:** The number to convert
 
-## Source
+## Sources
 
 - [Wikipedia - Roman numerals](https://en.wikipedia.org/wiki/Roman_numerals)
 - [Wikipedia - Arabic numerals](https://en.wikipedia.org/wiki/Arabic_numerals)
diff --git a/challenges/single-number/README.md b/challenges/single-number/README.md
new file mode 100644
index 0000000..756c3de
--- /dev/null
+++ b/challenges/single-number/README.md
@@ -0,0 +1,41 @@
+# 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
+```
diff --git a/challenges/single-number/solutions/.gitkeep b/challenges/single-number/solutions/.gitkeep
new file mode 100644
index 0000000..e69de29
diff --git a/challenges/single-number/test/1/input.txt b/challenges/single-number/test/1/input.txt
new file mode 100644
index 0000000..e710b72
--- /dev/null
+++ b/challenges/single-number/test/1/input.txt
@@ -0,0 +1,5 @@
+4
+1
+2
+1
+2
\ No newline at end of file
diff --git a/challenges/single-number/test/1/output.txt b/challenges/single-number/test/1/output.txt
new file mode 100644
index 0000000..bf0d87a
--- /dev/null
+++ b/challenges/single-number/test/1/output.txt
@@ -0,0 +1 @@
+4
\ No newline at end of file
diff --git a/challenges/single-number/test/2/input.txt b/challenges/single-number/test/2/input.txt
new file mode 100644
index 0000000..9359a2b
--- /dev/null
+++ b/challenges/single-number/test/2/input.txt
@@ -0,0 +1,3 @@
+2
+2
+1
\ No newline at end of file
diff --git a/challenges/single-number/test/2/output.txt b/challenges/single-number/test/2/output.txt
new file mode 100644
index 0000000..56a6051
--- /dev/null
+++ b/challenges/single-number/test/2/output.txt
@@ -0,0 +1 @@
+1
\ No newline at end of file
diff --git a/challenges/single-number/test/3/input.txt b/challenges/single-number/test/3/input.txt
new file mode 100644
index 0000000..56a6051
--- /dev/null
+++ b/challenges/single-number/test/3/input.txt
@@ -0,0 +1 @@
+1
\ No newline at end of file
diff --git a/challenges/single-number/test/3/output.txt b/challenges/single-number/test/3/output.txt
new file mode 100644
index 0000000..56a6051
--- /dev/null
+++ b/challenges/single-number/test/3/output.txt
@@ -0,0 +1 @@
+1
\ No newline at end of file
diff --git a/challenges/sorting-algorithms/README.md b/challenges/sorting-algorithms/README.md
index d7c2fea..472cecf 100644
--- a/challenges/sorting-algorithms/README.md
+++ b/challenges/sorting-algorithms/README.md
@@ -20,7 +20,7 @@ Write a function that takes a list of integers and sort them in ascending order.
 
 ## Source
 
-- [Wikipedia - Sorting algorithm](https://en.wikipedia.org/wiki/Sorting_algorithm)
+[Wikipedia - Sorting algorithm](https://en.wikipedia.org/wiki/Sorting_algorithm)
 
 ## Examples
 
diff --git a/challenges/sudoku/README.md b/challenges/sudoku/README.md
index 0f795a3..3ea112a 100644
--- a/challenges/sudoku/README.md
+++ b/challenges/sudoku/README.md
@@ -20,7 +20,7 @@ The empty cells are represented by 0.
 
 ## Source
 
-- [Wikipedia - Sudoku](https://en.wikipedia.org/wiki/Sudoku)
+[Wikipedia - Sudoku](https://en.wikipedia.org/wiki/Sudoku)
 
 ## Examples