fix(sorts): keep duplicate values in TreeSort

src/main/java/com/thealgorithms/sorts/TreeSort.java

@@ -3,6 +3,8 @@
 import static com.thealgorithms.sorts.SortUtils.print;
 
 import com.thealgorithms.datastructures.trees.BSTRecursiveGeneric;
+import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
 
 /**
@@ -30,38 +32,39 @@ public <T extends Comparable<T>> List<T> sort(List<T> unsortedList) {
     }
 
     private <T extends Comparable<T>> T[] doTreeSortArray(T[] unsortedArray) {
-        // create a generic BST tree
-        BSTRecursiveGeneric<T> tree = new BSTRecursiveGeneric<T>();
-
-        // add all elements to the tree
-        for (T element : unsortedArray) {
-            tree.add(element);
-        }
-
-        // get the sorted list by inorder traversal of the tree
-        List<T> sortedList = tree.inorderSort();
-
-        // add the elements back to the initial array
-        int i = 0;
-        for (T element : sortedList) {
-            unsortedArray[i++] = element;
-        }
-
-        // return the array
-        return unsortedArray;
+        // sort through the list path and copy the result back into the initial array
+        List<T> sortedList = doTreeSortList(Arrays.asList(unsortedArray));
+        return sortedList.toArray(unsortedArray);
     }
 
     private <T extends Comparable<T>> List<T> doTreeSortList(Iterable<T> unsortedList) {
-        // create a generic BST tree
-        BSTRecursiveGeneric<T> tree = new BSTRecursiveGeneric<T>();
+        // the BST ignores duplicate keys, so pair every element with its position to keep equal values apart
+        BSTRecursiveGeneric<Keyed<T>> tree = new BSTRecursiveGeneric<>();
 
         // add all elements to the tree
+        int position = 0;
         for (T element : unsortedList) {
-            tree.add(element);
+            tree.add(new Keyed<>(element, position++));
         }
 
         // get the sorted list by inorder traversal of the tree and return it
-        return tree.inorderSort();
+        List<T> sortedList = new ArrayList<>();
+        for (Keyed<T> keyed : tree.inorderSort()) {
+            sortedList.add(keyed.value());
+        }
+        return sortedList;
+    }
+
+    /**
+     * An element plus its original position. Equal elements are ordered by position,
+     * which also makes the sort stable.
+     */
+    private record Keyed<T extends Comparable<T>>(T value, int position) implements Comparable<Keyed<T>> {
+        @Override
+        public int compareTo(Keyed<T> other) {
+            int byValue = value.compareTo(other.value);
+            return byValue != 0 ? byValue : Integer.compare(position, other.position);
+        }
     }
 
     public static void main(String[] args) {

src/test/java/com/thealgorithms/sorts/TreeSortTest.java

@@ -9,9 +9,20 @@
  * @see TreeSort
  */
 
-public class TreeSortTest {
+public class TreeSortTest extends SortingAlgorithmTest {
     private TreeSort treeSort = new TreeSort();
 
+    @Override
+    SortAlgorithm getSortAlgorithm() {
+        return new TreeSort();
+    }
+
+    @Test
+    public void treeSortKeepsDuplicates() {
+        Integer[] array = {-1, -1, 0};
+        assertArrayEquals(new Integer[] {-1, -1, 0}, treeSort.sort(array));
+    }
+
     @Test
     public void treeSortEmptyArray() {
         Integer[] inputArray = {};