Fixed test file too for the duplicates change

Author: trynafind-sumanyu

src/test/java/com/thealgorithms/recursion/Permutations.java

@@ -2,32 +2,156 @@
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.util.List;
 import org.junit.jupiter.api.Test;
 
 class PermutationsTest {
 
+    // ─────────────────────────────────────────────
+    // Null / Invalid Input Tests
+    // ─────────────────────────────────────────────
+
+    @Test
+    void testNullArrayThrowsNullPointerException() {
+        assertThrows(NullPointerException.class, () -> Permutations.permutations((Integer[]) null));
+    }
+
+    @Test
+    void testArrayWithNullElementThrowsIllegalArgumentException() {
+        Integer[] items = { 1, null, 3 };
+        assertThrows(IllegalArgumentException.class, () -> Permutations.permutations(items));
+    }
+
+    // ─────────────────────────────────────────────
+    // Edge Case Tests
+    // ─────────────────────────────────────────────
+
+    @Test
+    void testEmptyArrayReturnsOneEmptyPermutation() {
+        Integer[] items = {};
+        List<List<Integer>> result = Permutations.permutations(items);
+        assertEquals(1, result.size());
+        assertTrue(result.get(0).isEmpty());
+    }
+
+    @Test
+    void testSingleElementReturnsOnePermutation() {
+        Integer[] items = { 42 };
+        List<List<Integer>> result = Permutations.permutations(items);
+        assertEquals(1, result.size());
+        assertEquals(List.of(42), result.get(0));
+    }
+
+    // ─────────────────────────────────────────────
+    // Integer Permutation Tests
+    // ─────────────────────────────────────────────
+
     @Test
-    void testIntegerPermutations() {
-        int[] nums = {1, 2};
-        List<List<Integer>> result = Permutations.permutations(nums);
+    void testTwoIntegersReturnsTwoPermutations() {
+        Integer[] items = { 1, 2 };
+        List<List<Integer>> result = Permutations.permutations(items);
         assertEquals(2, result.size());
+        assertTrue(result.contains(List.of(1, 2)));
+        assertTrue(result.contains(List.of(2, 1)));
     }
 
     @Test
-    void testIntegerPermutationsNull() {
-        assertThrows(NullPointerException.class, () -> Permutations.permutations((int[]) null));
+    void testThreeIntegersReturnsSixPermutations() {
+        Integer[] items = { 1, 2, 3 };
+        List<List<Integer>> result = Permutations.permutations(items);
+        assertEquals(6, result.size());
     }
 
     @Test
-    void testStringPermutations() {
-        List<String> result = Permutations.permutations("ab");
+    void testIntegerPermutationsContainAllExpectedOrders() {
+        Integer[] items = { 1, 2, 3 };
+        List<List<Integer>> result = Permutations.permutations(items);
+        assertTrue(result.contains(List.of(1, 2, 3)));
+        assertTrue(result.contains(List.of(1, 3, 2)));
+        assertTrue(result.contains(List.of(2, 1, 3)));
+        assertTrue(result.contains(List.of(2, 3, 1)));
+        assertTrue(result.contains(List.of(3, 1, 2)));
+        assertTrue(result.contains(List.of(3, 2, 1)));
+    }
+
+    // ─────────────────────────────────────────────
+    // Duplicate Handling Tests
+    // ─────────────────────────────────────────────
+
+    @Test
+    void testTwoDuplicateIntegersReturnsOnePermutation() {
+        Integer[] items = { 1, 1 };
+        List<List<Integer>> result = Permutations.permutations(items);
+        assertEquals(1, result.size());
+        assertEquals(List.of(1, 1), result.get(0));
+    }
+
+    @Test
+    void testArrayWithDuplicatesReturnsCorrectCount() {
+        Integer[] items = { 1, 1, 2 };
+        List<List<Integer>> result = Permutations.permutations(items);
+        // 3!/2! = 3 unique permutations
+        assertEquals(3, result.size());
+        assertTrue(result.contains(List.of(1, 1, 2)));
+        assertTrue(result.contains(List.of(1, 2, 1)));
+        assertTrue(result.contains(List.of(2, 1, 1)));
+    }
+
+    @Test
+    void testAllDuplicatesReturnsOnePermutation() {
+        Integer[] items = { 5, 5, 5 };
+        List<List<Integer>> result = Permutations.permutations(items);
+        assertEquals(1, result.size());
+        assertEquals(List.of(5, 5, 5), result.get(0));
+    }
+
+    // ─────────────────────────────────────────────
+    // String Permutation Tests
+    // ─────────────────────────────────────────────
+
+    @Test
+    void testTwoStringsReturnsTwoPermutations() {
+        String[] items = { "a", "b" };
+        List<List<String>> result = Permutations.permutations(items);
         assertEquals(2, result.size());
+        assertTrue(result.contains(List.of("a", "b")));
+        assertTrue(result.contains(List.of("b", "a")));
+    }
+
+    @Test
+    void testThreeStringsReturnsSixPermutations() {
+        String[] items = { "x", "y", "z" };
+        List<List<String>> result = Permutations.permutations(items);
+        assertEquals(6, result.size());
+    }
+
+    @Test
+    void testDuplicateStringsReturnsCorrectCount() {
+        String[] items = { "a", "a", "b" };
+        List<List<String>> result = Permutations.permutations(items);
+        assertEquals(3, result.size());
+        assertTrue(result.contains(List.of("a", "a", "b")));
+        assertTrue(result.contains(List.of("a", "b", "a")));
+        assertTrue(result.contains(List.of("b", "a", "a")));
+    }
+
+    // ─────────────────────────────────────────────
+    // Character Permutation Tests
+    // ─────────────────────────────────────────────
+
+    @Test
+    void testCharacterPermutations() {
+        Character[] items = { 'a', 'b', 'c' };
+        List<List<Character>> result = Permutations.permutations(items);
+        assertEquals(6, result.size());
     }
 
     @Test
-    void testStringPermutationsNull() {
-        assertThrows(NullPointerException.class, () -> Permutations.permutations((String) null));
+    void testDuplicateCharactersReturnsCorrectCount() {
+        Character[] items = { 'a', 'a', 'b' };
+        List<List<Character>> result = Permutations.permutations(items);
+        assertEquals(3, result.size());
     }
 }