Add Recaman's sequence (TheAlgorithms#250)

Co-authored-by: Valdas <[email protected]>

Author: Valdas3

Algorithms.Tests/Sequences/RecamansSequenceTests.cs

@@ -0,0 +1,29 @@
+using System.Linq;
+using System.Numerics;
+using Algorithms.Sequences;
+using FluentAssertions;
+using NUnit.Framework;
+
+namespace Algorithms.Tests.Sequences
+{
+    public class RecamansSequenceTests
+    {
+        [Test]
+        public void First50ElementsCorrect()
+        {
+            // Taken from http://oeis.org/A005132
+            var expected = new BigInteger[]
+            {
+                0, 1, 3, 6, 2, 7, 13, 20, 12, 21,
+                11, 22, 10, 23, 9, 24, 8, 25, 43, 62,
+                42, 63, 41, 18, 42, 17, 43, 16, 44, 15,
+                45, 14, 46, 79, 113, 78, 114, 77, 39, 78,
+                38, 79, 37, 80, 36, 81, 35, 82, 34, 83
+            };
+
+            var sequence = new RecamansSequence().Sequence.Take(50);
+
+            sequence.Should().Equal(expected);
+        }
+    }
+}

Algorithms/Sequences/RecamansSequence.cs

@@ -0,0 +1,47 @@
+using System.Collections.Generic;
+using System.Numerics;
+
+namespace Algorithms.Sequences
+{
+    /// <summary>
+    ///     <para>
+    ///         Recaman's sequence. a(0) = 0; for n > 0, a(n) = a(n-1) - n if nonnegative and not already in the sequence, otherwise a(n) = a(n-1) + n.
+    ///     </para>
+    ///     <para>
+    ///         Wikipedia: https://en.wikipedia.org/wiki/Recam%C3%A1n%27s_sequence.
+    ///     </para>
+    ///     <para>
+    ///         OEIS: http://oeis.org/A005132.
+    ///     </para>
+    /// </summary>
+    public class RecamansSequence : ISequence
+    {
+        /// <summary>
+        ///     Gets Recaman's sequence.
+        /// </summary>
+        public IEnumerable<BigInteger> Sequence
+        {
+            get
+            {
+                yield return 0;
+                var elements = new HashSet<BigInteger> { 0 };
+                var previous = 0;
+                var i = 1;
+
+                while (true)
+                {
+                    var current = previous - i;
+                    if (current < 0 || elements.Contains(current))
+                    {
+                        current = previous + i;
+                    }
+
+                    yield return current;
+                    previous = current;
+                    elements.Add(current);
+                    i++;
+                }
+            }
+        }
+    }
+}

README.md

@@ -90,6 +90,7 @@ This repository contains algorithms and data structures implemented in C# for ed
     * [A000040 Primes](./Algorithms/Sequences/PrimesSequence.cs)
     * [A000045 Fibonacci](./Algorithms/Sequences/FibonacciSequence.cs)
     * [A000142 Factorial](./Algorithms/Sequences/FactorialSequence.cs)
+    * [A005132 Recaman's](./Algorithms/Sequences/RecamansSequence.cs)
     * [A007318 Binomial](./Algorithms/Sequences/BinomialSequence.cs)
   * [String](./Algorithms/Strings)
     * [Longest Consecutive Character](./Algorithms/Strings/GeneralStringAlgorithms.cs)