Add permutations and combinations (#2932)

src/test/java/com/thealgorithms/backtracking/PermutationTest.java

@@ -0,0 +1,32 @@
+package com.thealgorithms.backtracking;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class PermutationTest {
+    @Test
+    void testNoElement()
+    {
+        List<Integer []> result = Permutation.permutation(new Integer[]{});
+        assertEquals(result.get(0).length, 0);
+    }
+    @Test
+    void testSingleElement()
+    {
+        List<Integer []> result = Permutation.permutation(new Integer[]{1});
+        assertEquals(result.get(0)[0], 1);
+    }
+    @Test
+    void testMultipleElements()
+    {
+        List<Integer []> result = Permutation.permutation(new Integer[]{1, 2});
+        assertTrue(Arrays.equals(result.get(0), new Integer[]{1,2}));
+        assertTrue(Arrays.equals(result.get(1), new Integer[]{2,1}));
+    }
+
+
+}