[Hacker Rank]: Project Euler #1: Multiples of 3 and 5 solved ✓

algorithm-exercises-csharp/src/hackerrank/projecteuler/Euler001.Test.cs

@@ -0,0 +1,29 @@
+namespace algorithm_exercises_csharp.hackerrank.prohecteuler;
+
+[TestClass]
+public class Euler001Test
+{
+  public class Euler001TestCase {
+    public int a; public int b; public int n; public int answer;
+  }
+
+  // dotnet_style_readonly_field = true
+  private static readonly Euler001TestCase[] tests = [
+    new() { a = 3, b = 5, n = 10, answer = 23},
+    new() { a = 3, b = 5, n = 100, answer = 2318},
+    new() { a = 3, b = 5, n = 1000, answer = 233168},
+
+  ];
+
+  [TestMethod]
+  public void Euler001ProblemTest()
+  {
+    int result;
+
+    foreach (Euler001TestCase test in tests) {
+      result = Euler001Problem.Euler001(test.a, test.b, test.n);
+        Assert.AreEqual(test.answer, result);
+    }
+  }
+}
+

algorithm-exercises-csharp/src/hackerrank/projecteuler/Euler001.cs

@@ -0,0 +1,36 @@
+namespace algorithm_exercises_csharp.hackerrank.prohecteuler;
+
+using System.Diagnostics.CodeAnalysis;
+
+public class Euler001Problem
+{
+  [ExcludeFromCodeCoverage]
+  protected Euler001Problem() {}
+
+  public static int SuOfArithmeticProgression(int n, int d) {
+    int new_n = n / d;
+
+    return new_n * (1 + new_n) * d / 2;
+  }
+
+  public static int GCD(int u, int v)
+  {
+    if (v != 0)
+    {
+        return GCD(v, u % v);
+    }
+    return u;
+  }
+
+  // Function to find the sum of all multiples of a and b below n
+  public static int Euler001(int a, int b, int n)
+  {
+    // Since, we need the sum of multiples less than N
+    int new_n = n - 1;
+    int lcm = a * b / GCD(a, b);
+
+    return SuOfArithmeticProgression(new_n, a) +
+      SuOfArithmeticProgression(new_n, b) -
+      SuOfArithmeticProgression(new_n, lcm);
+  }
+}