added UniquePermutations algorithm with test cases

Author: VarnitRaina

src/main/java/com/thealgorithms/Recursion/UniquePermutations.java

@@ -0,0 +1,34 @@
+package com.thealgorithms.Recursion;
+
+public class UniquePermutations {
+    throw new UnsupportedOperationException("Utility class");
+
+
+    public static List<String> getUniquePermutations(String str) {
+        List<String> result = new ArrayList<>();
+        boolean[] used = new boolean[str.length()];
+        char[] chars = str.toCharArray();
+        StringBuilder currentPermutation = new StringBuilder();
+        generatePermutations(chars, used, currentPermutation, result);
+        return result;
+    }
+
+    private static void generatePermutations(char[] chars, boolean[] used, StringBuilder currentPermutation, List<String> result) {
+        if (currentPermutation.length() == chars.length) {
+            result.add(currentPermutation.toString());
+            return;
+        }
+
+        for (int i = 0; i < chars.length; i++) {
+            if (used[i] || (i > 0 && chars[i] == chars[i - 1] && !used[i - 1])) {
+                continue;
+            }
+            used[i] = true;
+            currentPermutation.append(chars[i]);
+            generatePermutations(chars, used, currentPermutation, result);
+            used[i] = false;
+            currentPermutation.deleteCharAt(currentPermutation.length() - 1);
+        }
+    }
+}
+

src/test/java/com/thealgorithms/Recursion/UniquePermutationsTest.java

@@ -0,0 +1,41 @@
+package com.thealgorithms.recursion;
+
+import org.junit.jupiter.api.Test;
+import java.util.List;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class UniquePermutationsTest {
+
+    @Test
+    void testUniquePermutations() {
+        String str = "abc";
+        List<String> expected = List.of("abc", "acb", "bac", "bca", "cab", "cba");
+        List<String> result = UniquePermutations.getUniquePermutations(str);
+        assertEquals(expected.size(), result.size());
+        assertEquals(expected, result);
+    }
+
+    @Test
+    void testUniquePermutationsWithDuplicates() {
+        String str = "aab";
+        List<String> expected = List.of("aab", "aba", "baa");
+        List<String> result = UniquePermutations.getUniquePermutations(str);
+        assertEquals(expected.size(), result.size());
+        assertEquals(expected, result);
+    }
+
+    @Test
+    void testEmptyString() {
+        String str = "";
+        List<String> result = UniquePermutations.getUniquePermutations(str);
+        assertEquals(1, result.size()); // only 1 permutation which is an empty string
+    }
+
+    @Test
+    void testSingleCharacter() {
+        String str = "a";
+        List<String> result = UniquePermutations.getUniquePermutations(str);
+        assertEquals(1, result.size());
+        assertEquals("a", result.get(0));
+    }
+}