Add Levenshtein Distance (#336)

Co-authored-by: Valdas <valdas771@gmail.com>

Author: Valdas3

Algorithms.Tests/Strings/LevenshteinDistanceTests.cs

@@ -0,0 +1,21 @@
+using Algorithms.Strings;
+using NUnit.Framework;
+
+namespace Algorithms.Tests.Strings
+{
+    public class LevenshteinDistanceTests
+    {
+        [Test]
+        [TestCase("kitten", "sitting", 3)]
+        [TestCase("bob", "bond", 2)]
+        [TestCase("algorithm", "logarithm", 3)]
+        [TestCase("star", "", 4)]
+        [TestCase("", "star", 4)]
+        [TestCase("abcde", "12345", 5)]
+        public void Calculate_ReturnsCorrectLevenshteinDistance(string source, string destination, int expectedDistance)
+        {
+            var result = LevenshteinDistance.Calculate(source, destination);
+            Assert.AreEqual(expectedDistance, result);
+        }
+    }
+}

Algorithms/Strings/LevenshteinDistance.cs

@@ -0,0 +1,48 @@
+using System;
+
+namespace Algorithms.Strings
+{
+    /// <summary>
+    ///     <para>
+    ///         Levenshtein distance between two words is the minimum number of single-character edits (insertions, deletions or substitutions) required to change one word into the other.
+    ///     </para>
+    ///     <para>
+    ///         Wikipedia: https://en.wikipedia.org/wiki/Levenshtein_distance.
+    ///     </para>
+    /// </summary>
+    public static class LevenshteinDistance
+    {
+        /// <summary>
+        ///     Calculates Levenshtein distance.
+        ///     Time and space complexity is O(ab) where a and b are the lengths of the source and target strings.
+        /// </summary>
+        /// <param name="source">Source string.</param>
+        /// <param name="target">Target string.</param>
+        /// <returns>Levenshtein distance between source and target strings.</returns>
+        public static int Calculate(string source, string target)
+        {
+            var distances = new int[source.Length + 1, target.Length + 1];
+
+            for(var i = 0; i <= source.Length; i++)
+            {
+                distances[i, 0] = i;
+            }
+
+            for (var i = 0; i <= target.Length; i++)
+            {
+                distances[0, i] = i;
+            }
+
+            for (var i = 1; i <= source.Length; i++)
+            {
+                for (var j = 1; j <= target.Length; j++)
+                {
+                    var substitionCost = source[i - 1] == target[j - 1] ? 0 : 1;
+                    distances[i, j] = Math.Min(distances[i - 1, j] + 1, Math.Min(distances[i, j - 1] + 1, distances[i - 1, j - 1] + substitionCost));
+                }
+            }
+
+            return distances[source.Length, target.Length];
+        }
+    }
+}

README.md

@@ -158,6 +158,7 @@ find more than one implementation for the same objective but using different alg
     * [Boyer Moore](./Algorithms/Strings/BoyerMoore.cs)
     * [Palindrome Checker](./Algorithms/Strings/Palindrome.cs)
     * [Get all permutations of a string](./Algorithms/Strings/Permutation.cs)
+    * [Levenshtein Distance](./Algorithms/Strings/LevenshteinDistance.cs)
   * [Other](./Algorithms/Other)
     * [Fermat Prime Checker](./Algorithms/Other/FermatPrimeChecker.cs)
     * [Sieve of Eratosthenes](./Algorithms/Other/SieveOfEratosthenes.cs)