Catalan numbers

Algorithms/Algorithms.Tests/CatalanNumbersTests/CatalanNumbersTests.cs

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+using FluentAssertions;
+using Xunit;
+
+namespace Algorithms.Tests.CatalanNumbersTests
+{
+    public class CatalanNumbersTests
+    {
+        [Fact]
+        public void GetNumbers_Generate3rdCatalanNumber_Return5()
+        {
+            var expected = 5;
+
+            var number = CatalanNumberGenerator.GetNumberOn(3);
+
+            number.Should().Be(expected);
+        }
+
+        [Fact]
+        public void GetNumbers_Generate15rdCatalanNumber_Return9694845()
+        {
+            var expected = 9694845;
+
+            var number = CatalanNumberGenerator.GetNumberOn(15);
+
+            number.Should().Be(expected);
+        }
+
+        [Fact]
+        public void GetNumbers_Generate0rdCatalanNumber_Return1()
+        {
+            var expected = 0;
+
+            var number = CatalanNumberGenerator.GetNumberOn(0);
+
+            number.Should().Be(expected);
+        }
+
+        [Fact]
+        public void GetNumbers_GenerateNegativePos_Return0()
+        {
+            var expected = 0;
+
+            var number = CatalanNumberGenerator.GetNumberOn(-1);
+
+            number.Should().Be(expected);
+        }
+    }
+}

Algorithms/Algorithms/CatalanNumbers/CatalanNumberGenerator.cs

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+using System;
+
+namespace Algorithms
+{
+    public static class CatalanNumberGenerator
+    {
+        private static double Factorial(double n)
+        {
+            if (n <= 0.0)
+                return 1;
+
+            return n * Factorial(n - 1);
+        }
+
+        public static double GetNumberOn(int n)
+        {
+            if (n <= 0)
+                return 0;
+
+            const double topMultiplier = 2.0;
+            var result = Factorial(topMultiplier * n) / (Factorial(n + 1) * Factorial(n));
+
+            return Math.Ceiling(result);
+        }
+    }
+}