feat: kata/difference-of-squares (#791)

* docs: kata description

* feat: kata/difference-of-squares

---------

Co-authored-by: ParanoidUser <5120290+ParanoidUser@users.noreply.github.com>

Author: github-actions[bot]

kata/7-kyu/difference-of-squares/README.md

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+# [Difference Of Squares](https://www.codewars.com/kata/difference-of-squares "https://www.codewars.com/kata/558f9f51e85b46e9fa000025")
+
+*Recreation of [Project Euler problem #6](https://projecteuler.net/problem=6)*
+
+Find the difference between the square of the sum of the first `n` natural numbers `(1 <= n <= 100)` and the sum of their squares.
+
+## Example
+
+For example, when `n = 10`:
+
+* The square of the sum of the numbers is:
+
+(1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10)² = 55² = 3025
+
+* The sum of the squares of the numbers is:
+
+1² + 2² + 3² + 4² + 5² + 6² + 7² + 8² + 9² + 10² = 385
+
+Hence the difference between square of the sum of the first ten natural numbers and the sum of the squares of those numbers is:
+`3025 - 385 = 2640`

kata/7-kyu/difference-of-squares/main/Kata.java

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+interface Kata {
+  static int differenceOfSquares(int n) {
+    return (3 * n + 2) * (n * n * n - n) / 12;
+  }
+}

kata/7-kyu/difference-of-squares/test/DifferenceOfSquaresTest.java

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+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+
+class DifferenceOfSquaresTest {
+  @ParameterizedTest
+  @CsvSource(textBlock = """
+      5, 170
+      10, 2640
+      100, 25164150
+      """)
+  void sample(int n, int expected) {
+    assertEquals(expected, Kata.differenceOfSquares(n));
+  }
+}

kata/7-kyu/index.md

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 - [Dice Rotation](dice-rotation "5ff2093d375dca00170057bc")
 - [Difference between two collections](difference-between-two-collections "594093784aafb857f0000122")
 - [Difference between years. (Level 1)](difference-between-years-level-1 "588f5a38ec641b411200005b")
+- [Difference Of Squares](difference-of-squares "558f9f51e85b46e9fa000025")
 - [Digital cypher vol 2](digital-cypher-vol-2 "592edfda5be407b9640000b2")
 - [Digital cypher](digital-cypher "592e830e043b99888600002d")
 - [Digitize](digitize "5417423f9e2e6c2f040002ae")