Added threaded binary tree (#6995)

* Add One-Time Pad Cipher implementation in Java

* Add One-Time Pad Cipher implementation (fixed)

* Add ThreadedBinaryTree with in-order traversal and tests

* feat: add threaded binary tree implementation and tests

* fix: remove redundant null check for SpotBugs compliance

src/test/java/com/thealgorithms/datastructures/trees/ThreadedBinaryTreeTest.java

@@ -0,0 +1,50 @@
+/*
+ * TheAlgorithms (https://github.com/TheAlgorithms/Java)
+ * Author: Shewale41
+ * This file is licensed under the MIT License.
+ */
+
+package com.thealgorithms.datastructures.trees;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Basic tests for ThreadedBinaryTree inorder traversal.
+ */
+public class ThreadedBinaryTreeTest {
+
+    @Test
+    public void testInorderTraversalSimple() {
+        ThreadedBinaryTree tree = new ThreadedBinaryTree();
+        tree.insert(50);
+        tree.insert(30);
+        tree.insert(70);
+        tree.insert(20);
+        tree.insert(40);
+        tree.insert(60);
+        tree.insert(80);
+
+        List<Integer> expected = List.of(20, 30, 40, 50, 60, 70, 80);
+        List<Integer> actual = tree.inorderTraversal();
+
+        assertEquals(expected, actual);
+    }
+
+    @Test
+    public void testInorderWithDuplicates() {
+        ThreadedBinaryTree tree = new ThreadedBinaryTree();
+        tree.insert(5);
+        tree.insert(3);
+        tree.insert(7);
+        tree.insert(7); // duplicate
+        tree.insert(2);
+
+        List<Integer> expected = List.of(2, 3, 5, 7, 7);
+        List<Integer> actual = tree.inorderTraversal();
+
+        assertEquals(expected, actual);
+    }
+}