diff --git a/include/tree.h b/include/tree.h
index 8962f4b..8c70ce6 100644
--- a/include/tree.h
+++ b/include/tree.h
@@ -120,10 +120,9 @@ struct _tree
     // free
     void (*destory)(struct _tree* self);
 
-
     // ----- print -----
     // traversal depth
-    void (*preorder)(struct _tree* self);
+    void (*preorder)(struct _tree* self, struct _tree_node* root);
     void (*inorder)(struct _tree* self);
     void (*postorder)(struct _tree* self);
     // traversal breadth
@@ -131,11 +130,14 @@ struct _tree
 
     void (*print_obj)(void* obj);
 };
+typedef struct _tree* tree_t;
 
 // bst_tree
 bool tree_avl_init(struct _tree *self, uint32_t obj_size);
 bool tree_rb_init(struct _tree *self, uint32_t obj_size);
 
+tree_t tree_new(void);
+void tree_free(tree_t tree);
 
 #endif // _TREE_H_
 
diff --git a/src/tree.c b/src/tree.c
index 1f75ad8..e7045db 100644
--- a/src/tree.c
+++ b/src/tree.c
@@ -1404,6 +1404,8 @@ static bool tree_rebalance(ptree_t head, ptree_node_t tree)
 
 static tree_node_t tree_node_new(struct _tree* self, void* obj)
 {
+    assert(self != NULL);
+
     void * obj_new = malloc(self->_obj_size);
     if (obj_new == NULL)
     {
@@ -1437,12 +1439,14 @@ done:
 
 static bool tree_node_free(struct _tree_node* node)
 {
-    assert(node != NULL);
-    if(node->obj != NULL)
+    if(node != NULL)
     {
-        free(node->obj);
+        if(node->obj != NULL)
+        {
+            free(node->obj);
+        }
+        free(node);
     }
-    free(node);
     return true;
 }
 
@@ -1470,10 +1474,12 @@ bool tree_avl_insert(struct _tree* self, void* obj)
         if(self->compare(obj, root->obj) < 0)
         {
             root->left = node;
+            node->parent = root;
         }
         else if(self->compare(obj, root->obj) > 0)
         {
             root->right = node;
+            node->parent = root;
         }
         else
         {
@@ -1517,14 +1523,25 @@ void tree_destory(struct _tree* self)
     self->clear(self);
 }
 
-void tree_avl_preorder(struct _tree* self)
+void tree_avl_preorder(struct _tree* self, struct _tree_node* root)
 {
-
+    assert(self != NULL);
+    if(root->left != NULL)
+    {
+        tree_avl_preorder(self, root->left);
+    }
+    self->print_obj(root->obj);
+    if(root->right != NULL)
+    {
+        tree_avl_preorder(self, root->right);
+    }
 }
+
 void tree_avl_inorder(struct _tree* self)
 {
 
 }
+
 void tree_avl_postorder(struct _tree* self)
 {
 
@@ -1563,3 +1580,17 @@ bool tree_rb_init(struct _tree *self, uint32_t obj_size)
 {
 
 }
+
+tree_t tree_new(void)
+{
+    return (struct _tree*)malloc(sizeof(struct _tree));
+}
+
+void tree_free(tree_t tree)
+{
+    if(tree != NULL)
+    {
+        tree->destory(tree);
+        free(tree);
+    }
+}
diff --git a/test/test.c b/test/test.c
index 4699460..2b4e99f 100644
--- a/test/test.c
+++ b/test/test.c
@@ -6,6 +6,14 @@ void print_num(void* obj)
     printf("(%2d ) ", *(int*)obj);
 }
 
+int compare_num(void *obj, void *obj2)
+{
+    int num1 = *(int*)obj;
+    int num2 = *(int*)obj2;
+    return num1 - num2 > 0 ? 1 : -1;
+}
+
+
 void print_struct(void* obj)
 {
     struct _student* student = (struct _student*)obj;
@@ -27,12 +35,12 @@ int main()
     printf("----- unicstl test -----\n");
     // while (1)
     {	
-        test_list();
-        test_stack();
-        test_deque();
-        test_queue();
-        
-        // tree_test();
+        // test_list();
+        // test_stack();
+        // test_deque();
+        // test_queue();
+        test_tree();
+
         // rbtree_test();
     }
     return 0;
diff --git a/test/test.h b/test/test.h
index c5c079d..58c5da9 100644
--- a/test/test.h
+++ b/test/test.h
@@ -17,6 +17,8 @@ struct _student
 };
 
 void print_num(void* obj);
+int compare_num(void *obj, void *obj2);
+
 void print_char(void* obj);
 void print_struct(void* obj);
 void print_str(void* obj);
@@ -29,10 +31,8 @@ void print_str(void* obj);
 void test_list(void);
 void test_stack(void);
 void test_deque(void);
-
 void test_queue(void);
-void tree_test(void);
-void rbtree_test(void);
+void test_tree(void);
 
 #endif // _TEST_H_
 
diff --git a/test/tree_test.c b/test/tree_test.c
index 9f32092..a26a440 100644
--- a/test/tree_test.c
+++ b/test/tree_test.c
@@ -265,7 +265,35 @@ void tree_test(void)
     printf("----------------------------------------\n");
 }
 #endif
-
-
 #endif
 
+void test_tree_num(void)
+{
+    uint32_t i = 0;
+    int data[] = { 1,2,3,4,5,6,7,8,9,10 };
+    int temp = 0;
+    uint32_t len = sizeof(data) / sizeof(data[0]);
+
+    tree_t tree = tree_new();
+    tree_avl_init(tree, sizeof(int));
+    tree->print_obj = print_num;
+    tree->compare = compare_num;
+
+    printf("\n\n----- test_queue_num -----\n");
+
+    printf("----- after push-----\n");
+    for (i = 0; i < len; i++)
+    {
+        tree->insert(tree, &data[i]);
+    }
+    printf("----- preorder -----\n");
+    tree->preorder(tree, tree->_root);
+    printf("\n");
+
+    tree_free(tree);
+}
+
+void test_tree(void)
+{
+    test_tree_num();
+}