feat: Backtracking algorithms (All combinations) #3912 (#3917)

* ArrayCombination function which uses Combination.java by creating an array of 1 to n

* modified tests

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

@@ -0,0 +1,50 @@
+package com.thealgorithms.backtracking;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import java.util.List;
+import java.util.TreeSet;
+import org.junit.jupiter.api.Test;
+
+public class ArrayCombinationTest {
+
+    @Test
+    void testNBeingZeroOrLess() {
+        List<TreeSet<Integer>> zeroResult = ArrayCombination.combination(0, 1);
+        List<TreeSet<Integer>> negativeResult = ArrayCombination.combination(-1, 1);
+        assertNull(zeroResult);
+        assertNull(negativeResult);
+    }
+
+    @Test
+    void testNoLengthElement() {
+        List<TreeSet<Integer>> result = ArrayCombination.combination(2, 0);
+        assertNull(result);
+    }
+
+    @Test
+    void testLengthOne() {
+        List<TreeSet<Integer>> result = ArrayCombination.combination(2, 1);
+        assert result != null;
+        assertEquals(1, result.get(0).iterator().next());
+        assertEquals(2, result.get(1).iterator().next());
+    }
+
+    @Test
+    void testLengthTwo() {
+        List<TreeSet<Integer>> result = ArrayCombination.combination(2, 2);
+        assert result != null;
+        Integer[] arr = result.get(0).toArray(new Integer[2]);
+        assertEquals(1, arr[0]);
+        assertEquals(2, arr[1]);
+    }
+
+    @Test
+    void testLengthFive() {
+        List<TreeSet<Integer>> result = ArrayCombination.combination(10, 5);
+        assert result != null;
+        Integer[] arr = result.get(0).toArray(new Integer[5]);
+        assertEquals(1, arr[0]);
+        assertEquals(5, arr[4]);
+    }
+}