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

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

* modified tests

Author: TheClerici

src/main/java/com/thealgorithms/backtracking/ArrayCombination.java

@@ -0,0 +1,29 @@
+package com.thealgorithms.backtracking;
+
+import java.util.*;
+
+/**
+ * Finds all permutations of 1...n of length k
+ * @author TheClerici (https://github.com/TheClerici)
+ */
+public class ArrayCombination {
+    private static int length;
+
+    /**
+     * Find all combinations of 1..n by creating an array and using backtracking in Combination.java
+     * @param n max value of the array.
+     * @param k length of combination
+     * @return a list of all combinations of length k. If k == 0, return null.
+     */
+    public static List<TreeSet<Integer>> combination(int n, int k) {
+        if (n <= 0) {
+            return null;
+        }
+        length = k;
+        Integer[] arr = new Integer[n];
+        for (int i = 1; i <= n; i++) {
+            arr[i-1] = i;
+        }
+        return Combination.combination(arr, length);
+    }
+}

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]);
+    }
+}