diff --git a/src/createthread/CreateByCallable.java b/src/createthread/CreateByCallable.java
new file mode 100644
index 0000000..1fe5eba
--- /dev/null
+++ b/src/createthread/CreateByCallable.java
@@ -0,0 +1,22 @@
+package createthread;
+
+import java.util.concurrent.Callable;
+import java.util.concurrent.FutureTask;
+
+public class CreateByCallable implements Callable<String> {
+    @Override
+    public String call() throws Exception {
+        return "create by implementing Callable";
+    }
+
+    public static void main(String[] args) {
+        CreateByCallable createByCallable = new CreateByCallable();
+        FutureTask<String> futureTask = new FutureTask<>(createByCallable);
+        new Thread(futureTask).start();
+        try {
+            System.out.println(futureTask.get());
+        } catch (Exception e) {
+            e.printStackTrace();
+        }
+    }
+}
diff --git a/src/createthread/CreateByExecutorService.java b/src/createthread/CreateByExecutorService.java
new file mode 100644
index 0000000..0f775b7
--- /dev/null
+++ b/src/createthread/CreateByExecutorService.java
@@ -0,0 +1,27 @@
+package createthread;
+
+import java.util.concurrent.*;
+
+public class CreateByExecutorService {
+
+    public static void main(String[] args) {
+        ExecutorService poolA = Executors.newFixedThreadPool(2);
+        poolA.execute(() -> {
+            System.out.println("poolA");
+        });
+
+        poolA.shutdown();
+
+        ThreadPoolExecutor poolB = new ThreadPoolExecutor(2,3,0,
+                TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>(3),
+                Executors.defaultThreadFactory(), new ThreadPoolExecutor.AbortPolicy());
+
+        poolB.submit(() -> {
+            System.out.println("poolB");
+        });
+        poolB.shutdown();
+
+    }
+
+
+}
diff --git a/src/createthread/CreateByRunnable.java b/src/createthread/CreateByRunnable.java
new file mode 100644
index 0000000..5948c5e
--- /dev/null
+++ b/src/createthread/CreateByRunnable.java
@@ -0,0 +1,13 @@
+package createthread;
+
+public class CreateByRunnable implements Runnable{
+    @Override
+    public void run() {
+        System.out.println("create by implementing Runnable");
+    }
+
+    public static void main(String[] args) {
+        CreateByRunnable createByRunnable = new CreateByRunnable();
+        new Thread(createByRunnable).start();
+    }
+}
diff --git a/src/createthread/CreateByThread.java b/src/createthread/CreateByThread.java
new file mode 100644
index 0000000..5bab5b2
--- /dev/null
+++ b/src/createthread/CreateByThread.java
@@ -0,0 +1,13 @@
+package createthread;
+
+public class CreateByThread extends Thread{
+    @Override
+    public void run(){
+        System.out.println("create new thread by extending Thread");
+    }
+
+    public static void main(String[] args) {
+        CreateByThread createByThread = new CreateByThread();
+        createByThread.start();
+    }
+}
diff --git a/src/recursive/TreeNode.java b/src/recursive/TreeNode.java
new file mode 100644
index 0000000..be85746
--- /dev/null
+++ b/src/recursive/TreeNode.java
@@ -0,0 +1,43 @@
+package recursive;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class TreeNode {
+
+    private final int id;
+
+    private final int parentId;
+
+    private final String name;
+
+    private final List<TreeNode> children;
+
+    public TreeNode(int id, int parentId, String name) {
+        this.id = id;
+        this.parentId = parentId;
+        this.name = name;
+        children = new ArrayList<>();
+    }
+
+    public void addChild(TreeNode child) {
+        children.add(child);
+    }
+
+    public int getId() {
+        return id;
+    }
+
+    public int getParentId() {
+        return parentId;
+    }
+
+    @Override
+    public String toString() {
+        return "TreeNode{" +
+                "id=" + id +
+                ", parentId=" + parentId +
+                ", children=" + children.toString() +
+                '}';
+    }
+}
diff --git a/src/recursive/TreeStructure.java b/src/recursive/TreeStructure.java
new file mode 100644
index 0000000..10fb3c0
--- /dev/null
+++ b/src/recursive/TreeStructure.java
@@ -0,0 +1,42 @@
+package recursive;
+
+import java.util.*;
+
+public class TreeStructure {
+
+    public static void main(String[] args) {
+        List<TreeNode> treeNodes = new ArrayList<>();
+        Set<Integer> leafSet = new HashSet<>();
+        treeNodes.add(new TreeNode(1,0,"root"));
+        treeNodes.add(new TreeNode(2,1,"a1"));
+        treeNodes.add(new TreeNode(3,1,"a2"));
+        treeNodes.add(new TreeNode(4,2,"b1"));
+        treeNodes.add(new TreeNode(5,4,"c1"));
+
+        Map<Integer, TreeNode> treeMap = new HashMap<>();
+
+        for (TreeNode treeNode : treeNodes) {
+            leafSet.add(treeNode.getId());
+        }
+
+        for (TreeNode treeNode : treeNodes) {
+            treeMap.put(treeNode.getId(), treeNode);
+            leafSet.remove(treeNode.getParentId());
+        }
+
+        for (Integer id: leafSet) {
+            int parentId = treeMap.get(id).getParentId();
+            int nodeId = id;
+            while (parentId != 0) {
+                treeMap.get(parentId).addChild(treeMap.get(nodeId));
+                nodeId = parentId;
+                parentId = treeMap.get(nodeId).getParentId();
+            }
+        }
+
+        TreeNode finalNode = treeMap.values().stream()
+                .filter(treeNode -> treeNode.getParentId() == 0).toList().getFirst();
+
+        System.out.println(finalNode);
+    }
+}