refactor: ReturnSubsequence (#5408)

* refactor: ReturnSubsequence

* checkstyle: fix formatting

* checkstyle: fix formatting

---------

Co-authored-by: alxkm <alx@alx.com>

src/main/java/com/thealgorithms/others/ReturnSubsequence.java

@@ -1,46 +1,37 @@
 package com.thealgorithms.others;
 
-import java.util.Scanner;
-
+/**
+ * Class for generating all subsequences of a given string.
+ */
 public final class ReturnSubsequence {
     private ReturnSubsequence() {
     }
 
-    public static void main(String[] args) {
-        System.out.println("Enter String: ");
-        Scanner s = new Scanner(System.in);
-        String givenString = s.next(); // given string
-        String[] subsequence = returnSubsequence(givenString); // calling returnSubsequence() function
-        System.out.println("Subsequences : ");
-        // print the given array of subsequences
-        for (int i = 0; i < subsequence.length; i++) {
-            System.out.println(subsequence[i]);
-        }
-        s.close();
-    }
-
     /**
-     * @param givenString
-     * @return subsequence
+     * Generates all subsequences of the given string.
+     *
+     * @param input The input string.
+     * @return An array of subsequences.
      */
-    private static String[] returnSubsequence(String givenString) {
-        if (givenString.length() == 0) { // in it // If string is empty we will create an array of
-                                         // size=1 and insert "" (Empty string)
-            String[] ans = new String[1];
-            ans[0] = "";
-            return ans;
+    public static String[] getSubsequences(String input) {
+        if (input.isEmpty()) {
+            return new String[] {""}; // Return array with an empty string if input is empty
         }
-        String[] smallAns = returnSubsequence(givenString.substring(1)); // recursive call to get subsequences of substring starting from index
-        // position=1
 
-        String[] ans = new String[2 * smallAns.length]; // Our answer will be an array off string of size=2*smallAns
-        System.arraycopy(smallAns, 0, ans, 0, smallAns.length);
+        // Recursively find subsequences of the substring (excluding the first character)
+        String[] smallerSubsequences = getSubsequences(input.substring(1));
 
-        for (int k = 0; k < smallAns.length; k++) {
-            ans[k + smallAns.length] = givenString.charAt(0) + smallAns[k]; // Insert character at index=0 of the given
-                                                                            // substring in front of every string
-            // in smallAns
+        // Create an array to hold the final subsequences, double the size of smallerSubsequences
+        String[] result = new String[2 * smallerSubsequences.length];
+
+        // Copy the smaller subsequences directly to the result array
+        System.arraycopy(smallerSubsequences, 0, result, 0, smallerSubsequences.length);
+
+        // Prepend the first character of the input string to each of the smaller subsequences
+        for (int i = 0; i < smallerSubsequences.length; i++) {
+            result[i + smallerSubsequences.length] = input.charAt(0) + smallerSubsequences[i];
         }
-        return ans;
+
+        return result;
     }
 }

src/test/java/com/thealgorithms/others/ReturnSubsequenceTest.java

@@ -0,0 +1,23 @@
+package com.thealgorithms.others;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class ReturnSubsequenceTest {
+
+    @ParameterizedTest
+    @MethodSource("provideTestCases")
+    void testSubsequences(String input, String[] expected) {
+        String[] actual = ReturnSubsequence.getSubsequences(input);
+        assertArrayEquals(expected, actual);
+    }
+
+    static Stream<Arguments> provideTestCases() {
+        return Stream.of(Arguments.of("", new String[] {""}), Arguments.of("a", new String[] {"", "a"}), Arguments.of("ab", new String[] {"", "b", "a", "ab"}), Arguments.of("abc", new String[] {"", "c", "b", "bc", "a", "ac", "ab", "abc"}),
+            Arguments.of("aab", new String[] {"", "b", "a", "ab", "a", "ab", "aa", "aab"}));
+    }
+}