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

Author: Shewale41

src/main/java/com/thealgorithms/datastructures/trees/ThreadedBinaryTree.java

@@ -0,0 +1,145 @@
+/*
+ * TheAlgorithms (https://github.com/TheAlgorithms/Java)
+ * Author: Shewale41
+ * This file is licensed under the MIT License.
+ */
+
+package com.thealgorithms.datastructures.trees;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Threaded binary tree implementation that supports insertion and
+ * in-order traversal without recursion or stack by using threads.
+ *
+ * <p>In this implementation, a node's null left/right pointers are used
+ * to point to the in-order predecessor/successor respectively. Two flags
+ * indicate whether left/right pointers are real children or threads.
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Threaded_binary_tree">Wikipedia:
+ * Threaded binary tree</a>
+ */
+public final class ThreadedBinaryTree {
+
+    private Node root;
+
+    private static final class Node {
+        int value;
+        Node left;
+        Node right;
+        boolean leftIsThread;
+        boolean rightIsThread;
+
+        Node(int value) {
+            this.value = value;
+            this.left = null;
+            this.right = null;
+            this.leftIsThread = false;
+            this.rightIsThread = false;
+        }
+    }
+
+    public ThreadedBinaryTree() {
+        this.root = null;
+    }
+
+    /**
+     * Inserts a value into the threaded binary tree. Duplicate values are inserted
+     * to the right subtree (consistent deterministic rule).
+     *
+     * @param value the integer value to insert
+     */
+    public void insert(int value) {
+        Node newNode = new Node(value);
+        if (root == null) {
+            root = newNode;
+            return;
+        }
+
+        Node current = root;
+        Node parent = null;
+
+        while (true) {
+            parent = current;
+            if (value < current.value) {
+                if (!current.leftIsThread && current.left != null) {
+                    current = current.left;
+                } else {
+                    break;
+                }
+            } else { // value >= current.value
+                if (!current.rightIsThread && current.right != null) {
+                    current = current.right;
+                } else {
+                    break;
+                }
+            }
+        }
+
+        if (value < parent.value) {
+            // attach newNode as left child
+            newNode.left = parent.left;
+            newNode.leftIsThread = parent.leftIsThread;
+            newNode.right = parent;
+            newNode.rightIsThread = true;
+
+            parent.left = newNode;
+            parent.leftIsThread = false;
+        } else {
+            // attach newNode as right child
+            newNode.right = parent.right;
+            newNode.rightIsThread = parent.rightIsThread;
+            newNode.left = parent;
+            newNode.leftIsThread = true;
+
+            parent.right = newNode;
+            parent.rightIsThread = false;
+        }
+    }
+
+    /**
+     * Returns the in-order traversal of the tree as a list of integers.
+     * Traversal is done without recursion or an explicit stack by following threads.
+     *
+     * @return list containing the in-order sequence of node values
+     */
+    public List<Integer> inorderTraversal() {
+        List<Integer> result = new ArrayList<>();
+        Node current = root;
+        if (current == null) {
+            return result;
+        }
+
+        // Move to the leftmost node
+        while (current.left != null && !current.leftIsThread) {
+            current = current.left;
+        }
+
+        while (current != null) {
+            result.add(current.value);
+
+            // If right pointer is a thread, follow it
+            if (current.rightIsThread) {
+                current = current.right;
+            } else {
+                // Move to leftmost node in right subtree
+                current = current.right;
+                while (current != null && !current.leftIsThread && current.left != null) {
+                    current = current.left;
+                }
+            }
+        }
+
+        return result;
+    }
+
+    /**
+     * Helper: checks whether the tree is empty.
+     *
+     * @return true if tree has no nodes
+     */
+    public boolean isEmpty() {
+        return root == null;
+    }
+}

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

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+/*
+ * 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);
+    }
+}