Added Project Euler's problem 10 (#110)

* feat:add Project Euler problem 10

* chore:add problem in DIRECTORY

Author: rubendelblanco

DIRECTORY.md

@@ -45,6 +45,7 @@
     * [Problem7](./Maths/ProjectEuler/Problem7.php)
     * [Problem8](./Maths/ProjectEuler/Problem8.php)
     * [Problem9](./Maths/ProjectEuler/Problem9.php)
+    * [Problem10](./Maths/ProjectEuler/Problem10.php)
 
 ## Searches
   * [Binarysearch](./Searches/BinarySearch.php)

Maths/ProjectEuler/Problem10.php

@@ -0,0 +1,33 @@
+<?php
+/*
+ * This function solves the problem 10 of the Project Euler.
+ *
+ * Problem description:
+ * The sum of the primes below 10 is 2 + 3 + 5 + 7 = 17.
+ * Find the sum of all the primes below two million.
+ */
+
+/**
+ * @return int
+ */
+function problem10(): int
+{
+    //Using Sieve of Erastothenes
+    //@see https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
+
+    $n = 2000000;
+    $isPrime = [];
+
+    for ($i = 2; $i <= $n; $i++){
+        $isPrime[$i] = $i;
+    }
+
+    for ($i = 2; $i*$i <= $n; $i++){
+        if (isset ($isPrime[$i])){
+            for ( $j = $i; $i*$j <= $n; $j++)
+               unset($isPrime[$i*$j]);
+        }
+    }
+
+    return array_sum($isPrime);
+}

tests/Maths/ProjectEulerTest.php

@@ -12,6 +12,7 @@
 require_once __DIR__ . '/../../Maths/ProjectEuler/Problem7.php';
 require_once __DIR__ . '/../../Maths/ProjectEuler/Problem8.php';
 require_once __DIR__ . '/../../Maths/ProjectEuler/Problem9.php';
+require_once __DIR__ . '/../../Maths/ProjectEuler/Problem10.php';
 
 class ProjectEulerTest extends TestCase
 {
@@ -61,4 +62,9 @@ public function testProblem9(): void
         $this->assertSame(31875000, problem9());
     }
 
+    public function testProblem10(): void
+    {
+        $this->assertSame(142913828922, problem10());
+    }
+
 }