BAEL-5646: String permutation improvements (#12613)

algorithms-modules/algorithms-miscellaneous-4/src/main/java/com/baeldung/algorithms/stringpermutation/Helper.java

@@ -1,14 +1,20 @@
 package com.baeldung.algorithms.stringpermutation;
 
+import java.util.Collection;
 import java.util.List;
 import java.util.stream.Collectors;
 
-public class ArrayHelper {
+public class Helper {
 
-    private ArrayHelper() {
+    private Helper() {
     }
 
     static List<Character> toCharacterList(final String string) {
         return string.chars().mapToObj(s -> ((char) s)).collect(Collectors.toList());
     }
+
+    static String toString(Collection<Character> collection) {
+        return collection.stream().map(Object::toString).collect(Collectors.joining());
+    }
+
 }

algorithms-modules/algorithms-miscellaneous-4/src/main/java/com/baeldung/algorithms/stringpermutation/StringPermutationsApache.java

@@ -1,20 +1,29 @@
 package com.baeldung.algorithms.stringpermutation;
 
-import java.util.Collection;
+import java.util.ArrayList;
 import java.util.List;
+import java.util.stream.Collectors;
 import org.apache.commons.collections4.CollectionUtils;
 import org.apache.commons.collections4.iterators.PermutationIterator;
 
 public class StringPermutationsApache {
 
-    public Collection<List<Character>> eagerPermutationWithRepetitions(final String string) {
-        final List<Character> characters = ArrayHelper.toCharacterList(string);
-        return CollectionUtils.permutations(characters);
+    public List<String> eagerPermutationWithRepetitions(final String string) {
+        final List<Character> characters = Helper.toCharacterList(string);
+        return CollectionUtils.permutations(characters)
+            .stream()
+            .map(Helper::toString)
+            .collect(Collectors.toList());
     }
 
-    public PermutationIterator<Character> lazyPermutationWithoutRepetitions(final String string) {
-        final List<Character> characters = ArrayHelper.toCharacterList(string);
-        return new PermutationIterator<>(characters);
+    public List<String> lazyPermutationWithoutRepetitions(final String string) {
+        final List<Character> characters = Helper.toCharacterList(string);
+        final PermutationIterator<Character> permutationIterator = new PermutationIterator<>(characters);
+        final List<String> result = new ArrayList<>();
+        while (permutationIterator.hasNext()) {
+            result.add(Helper.toString(permutationIterator.next()));
+        }
+        return result;
     }
 
 }

algorithms-modules/algorithms-miscellaneous-4/src/main/java/com/baeldung/algorithms/stringpermutation/StringPermutationsCombinatoricsLib.java

@@ -7,19 +7,21 @@ import org.paukov.combinatorics3.PermutationGenerator.TreatDuplicatesAs;
 
 public class StringPermutationsCombinatoricsLib {
 
-    public List<List<Character>> permutationWithoutRepetitions(final String string) {
-        List<Character> chars = ArrayHelper.toCharacterList(string);
-        return Generator.permutation(chars)
-            .simple()
-            .stream()
-            .collect(Collectors.toList());
-    }
+public List<String> permutationWithoutRepetitions(final String string) {
+    List<Character> chars = Helper.toCharacterList(string);
+    return Generator.permutation(chars)
+        .simple()
+        .stream()
+        .map(Helper::toString)
+        .collect(Collectors.toList());
+}
 
-    public List<List<Character>> permutationWithRepetitions(final String string) {
-        List<Character> chars = ArrayHelper.toCharacterList(string);
+    public List<String> permutationWithRepetitions(final String string) {
+        List<Character> chars = Helper.toCharacterList(string);
         return Generator.permutation(chars)
             .simple(TreatDuplicatesAs.IDENTICAL)
             .stream()
+            .map(Helper::toString)
             .collect(Collectors.toList());
     }
 

algorithms-modules/algorithms-miscellaneous-4/src/main/java/com/baeldung/algorithms/stringpermutation/StringPermutationsGuava.java

@@ -1,18 +1,22 @@
 package com.baeldung.algorithms.stringpermutation;
 
 import com.google.common.collect.Collections2;
-import java.util.Collection;
 import java.util.List;
+import java.util.stream.Collectors;
 
 public class StringPermutationsGuava {
 
-    public Collection<List<Character>> permutationWithRepetitions(final String string) {
-        final List<Character> characters = ArrayHelper.toCharacterList(string);
-        return Collections2.permutations(characters);
+    public List<String> permutationWithRepetitions(final String string) {
+        final List<Character> characters = Helper.toCharacterList(string);
+        return Collections2.permutations(characters).stream()
+            .map(Helper::toString)
+            .collect(Collectors.toList());
+    }
+    public List<String> permutationWithoutRepetitions(final String string) {
+        final List<Character> characters = Helper.toCharacterList(string);
+        return Collections2.orderedPermutations(characters).stream()
+            .map(Helper::toString)
+            .collect(Collectors.toList());
     }
-public Collection<List<Character>> permutationWithoutRepetitions(final String string) {
-    final List<Character> characters = ArrayHelper.toCharacterList(string);
-    return Collections2.orderedPermutations(characters);
-}
 
 }