fix variation

Author: justcoding121

Algorithm.Sandbox.Tests/Combinatorics/Variation_Tests.cs

@@ -30,26 +30,26 @@ public void Variation_Without_Repetitions_Smoke_Test()
             Variations = Variation.Find<char>(input, 3, false);
             Assert.AreEqual(combination(input.Count, 3) * factorial(3), Variations.Count);
 
-            input = "monster".ToCharArray().ToList();
-            Variations = Variation.Find<char>(input, 4, false);
-            Assert.AreEqual(combination(input.Count, 4) * factorial(4), Variations.Count);
+            input = "monsters".ToCharArray().ToList();
+            Variations = Variation.Find<char>(input, 3, false);
+            Assert.AreEqual(combination(input.Count, 3) * factorial(3), Variations.Count);
         }
 
 
         [TestMethod]
         public void Variation_With_Repetitions_Smoke_Test()
         {
-            var input = "".ToCharArray().ToList();
-            var variations = Variation.Find<char>(input, 3, true);
-            Assert.AreEqual(0, variations.Count);
-
-            input = "abc".ToCharArray().ToList();
-            variations = Variation.Find<char>(input, 2, true);
+            var input = "abcd".ToCharArray().ToList();
+            var variations = Variation.Find<char>(input, 2, true);
             Assert.AreEqual(Math.Pow(input.Count, 2), variations.Count);
 
             input = "scan".ToCharArray().ToList();
             variations = Variation.Find<char>(input, 3, true);
             Assert.AreEqual(Math.Pow(input.Count, 3), variations.Count);
+
+            input = "".ToCharArray().ToList();
+            variations = Variation.Find<char>(input, 3, true);
+            Assert.AreEqual(0, variations.Count);
         }
 
     }

Algorithm.Sandbox/Combinatorics/Combination.cs

@@ -26,10 +26,6 @@ private static void Recurse<T>(List<T> input, int r, bool enableRepetition,
                 result.Add(new List<T>(prefix));
                 return;
             }
-            else
-            {
-
-            }
 
             for (int j = k; j < input.Count; j++)
             {

Algorithm.Sandbox/Combinatorics/Variation.cs

@@ -22,20 +22,36 @@ public static List<List<T>> Find<T>(List<T> input, int r, bool enableRepetition)
         {
             var result = new List<List<T>>();
 
-            var combinations = Combination.Find(input, r, false);
+            Recurse(input, r, enableRepetition, new List<T>(), new HashSet<int>(), result);
 
-            foreach (var combination in combinations)
+            return result;
+        }
+
+        private static void Recurse<T>(List<T> input, int r, bool enableRepetition,
+         List<T> prefix, HashSet<int> prefixIndices,
+         List<List<T>> result)
+        {
+            if (prefix.Count == r)
             {
-                var permutations = Permutation.Find(combination, enableRepetition);
+                result.Add(new List<T>(prefix));
+                return;
+            }
 
-                foreach(var permutation in permutations)
+            for (int j = 0; j < input.Count; j++)
+            {
+                if (prefixIndices.Contains(j) && !enableRepetition)
                 {
-                    result.Add(permutation);
+                    continue;
                 }
-                
-            }
 
-            return result;
+                prefix.Add(input[j]);
+                prefixIndices.Add(j);
+
+                Recurse(input, r, enableRepetition, prefix, prefixIndices, result);
+
+                prefix.RemoveAt(prefix.Count - 1);
+                prefixIndices.Remove(j);
+            }
         }
     }
 }