diff --git a/demo/demo_tree.c b/demo/demo_tree.c
index 79822e4..e9313c7 100644
--- a/demo/demo_tree.c
+++ b/demo/demo_tree.c
@@ -55,7 +55,7 @@ void demo_avltree_num(void)
         tree->print_obj(&temp);
         printf("size = %2d : ", tree->size(tree));
 
-        iter = tree->iter(tree, ORDER_PRE);
+        iter = tree->iter(tree, TREE_DFS_PRE);
         while(iter->hasnext(iter))
         {
             temp = *(int *)iter->next(iter);
@@ -88,7 +88,7 @@ void demo_avltree_num(void)
     }
 
     printf("----- preorder -----\n");
-    iter = tree->iter(tree, ORDER_PRE);
+    iter = tree->iter(tree, TREE_DFS_PRE);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -97,7 +97,7 @@ void demo_avltree_num(void)
     printf("\n");
 
     printf("----- inorder -----\n");
-    iter = tree->iter(tree, ORDER_IN);
+    iter = tree->iter(tree, TREE_DFS_IN);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -106,7 +106,7 @@ void demo_avltree_num(void)
     printf("\n");
 
     printf("----- postorder -----\n");
-    iter = tree->iter(tree, ORDER_POST);
+    iter = tree->iter(tree, TREE_DFS_POST);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -115,7 +115,7 @@ void demo_avltree_num(void)
     printf("\n");
 
     printf("----- breadth -----\n");
-    iter = tree->iter(tree, ORDER_BREADTH);
+    iter = tree->iter(tree, TREE_BFS);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -124,7 +124,7 @@ void demo_avltree_num(void)
     printf("\n");
 
     printf("----- preorder(right) -----\n");
-    iter = tree->iter(tree, ORDER_PRE_R);
+    iter = tree->iter(tree, TREE_DFS_PRE_R);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -133,7 +133,7 @@ void demo_avltree_num(void)
     printf("\n");
 
     printf("----- inorder(right) -----\n");
-    iter = tree->iter(tree, ORDER_IN_R);
+    iter = tree->iter(tree, TREE_DFS_IN_R);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -142,7 +142,7 @@ void demo_avltree_num(void)
     printf("\n");
 
     printf("----- postorder(right) -----\n");
-    iter = tree->iter(tree, ORDER_POST_R);
+    iter = tree->iter(tree, TREE_DFS_POST_R);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -151,7 +151,7 @@ void demo_avltree_num(void)
     printf("\n");
 
     printf("----- breadth(right) -----\n");
-    iter = tree->iter(tree, ORDER_BREADTH_R);
+    iter = tree->iter(tree, TREE_BFS_R);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -161,7 +161,7 @@ void demo_avltree_num(void)
 
 
     printf("----- preorder(left) -----\n");
-    iter = tree->iter(tree, ORDER_PRE);
+    iter = tree->iter(tree, TREE_DFS_PRE);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -179,7 +179,7 @@ void demo_avltree_num(void)
         tree->print_obj(&temp);
         printf("size = %2d : ", tree->size(tree));
 
-        iter = tree->iter(tree, ORDER_PRE);
+        iter = tree->iter(tree, TREE_DFS_PRE);
         while(iter->hasnext(iter))
         {
             temp = *(int *)iter->next(iter);
@@ -279,7 +279,7 @@ void demo_rbtree_num(void)
         tree->print_obj(&temp);
         printf("size = %2d : ", tree->size(tree));
 
-        iter = tree->iter(tree, ORDER_PRE);
+        iter = tree->iter(tree, TREE_DFS_PRE);
         while(iter->hasnext(iter))
         {
             temp = *(int *)iter->next(iter);
@@ -318,7 +318,7 @@ void demo_rbtree_num(void)
     }
 
     printf("----- preorder -----\n");
-    iter = tree->iter(tree, ORDER_PRE);
+    iter = tree->iter(tree, TREE_DFS_PRE);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -327,7 +327,7 @@ void demo_rbtree_num(void)
     printf("\n");
 
     printf("----- inorder -----\n");
-    iter = tree->iter(tree, ORDER_IN);
+    iter = tree->iter(tree, TREE_DFS_IN);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -336,7 +336,7 @@ void demo_rbtree_num(void)
     printf("\n");
 
     printf("----- postorder -----\n");
-    iter = tree->iter(tree, ORDER_POST);
+    iter = tree->iter(tree, TREE_DFS_POST);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -345,7 +345,7 @@ void demo_rbtree_num(void)
     printf("\n");
 
     printf("----- breadth -----\n");
-    iter = tree->iter(tree, ORDER_BREADTH);
+    iter = tree->iter(tree, TREE_BFS);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -354,7 +354,7 @@ void demo_rbtree_num(void)
     printf("\n");
 
     printf("----- preorder(right) -----\n");
-    iter = tree->iter(tree, ORDER_PRE_R);
+    iter = tree->iter(tree, TREE_DFS_PRE_R);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -363,7 +363,7 @@ void demo_rbtree_num(void)
     printf("\n");
 
     printf("----- inorder(right) -----\n");
-    iter = tree->iter(tree, ORDER_IN_R);
+    iter = tree->iter(tree, TREE_DFS_IN_R);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -372,7 +372,7 @@ void demo_rbtree_num(void)
     printf("\n");
 
     printf("----- postorder(right) -----\n");
-    iter = tree->iter(tree, ORDER_POST_R);
+    iter = tree->iter(tree, TREE_DFS_POST_R);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -381,7 +381,7 @@ void demo_rbtree_num(void)
     printf("\n");
 
     printf("----- breadth(right) -----\n");
-    iter = tree->iter(tree, ORDER_BREADTH_R);
+    iter = tree->iter(tree, TREE_BFS_R);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -390,7 +390,7 @@ void demo_rbtree_num(void)
     printf("\n");
 
     printf("----- preorder -----\n");
-    iter = tree->iter(tree, ORDER_PRE);
+    iter = tree->iter(tree, TREE_DFS_PRE);
     while(iter->hasnext(iter))
     {
         temp = *(int *)iter->next(iter);
@@ -409,7 +409,7 @@ void demo_rbtree_num(void)
 
         printf("size = %2d : ", tree->size(tree));
 
-        iter = tree->iter(tree, ORDER_PRE);
+        iter = tree->iter(tree, TREE_DFS_PRE);
         while(iter->hasnext(iter))
         {
             temp = *(int *)iter->next(iter);
@@ -461,7 +461,7 @@ void demo_rbtree_struct(void)
         tree->print_obj(&temp);
         printf("size = %2d : ", tree->size(tree));
         
-        iter = tree->iter(tree, ORDER_PRE);
+        iter = tree->iter(tree, TREE_DFS_PRE);
         while(iter->hasnext(iter))
         {
             temp = *(struct _student *)iter->next(iter);
@@ -500,7 +500,7 @@ void demo_rbtree_struct(void)
     }
 
     printf("----- preorder -----\n");
-    iter = tree->iter(tree, ORDER_PRE);
+    iter = tree->iter(tree, TREE_DFS_PRE);
     while(iter->hasnext(iter))
     {
         temp = *(struct _student *)iter->next(iter);
@@ -509,7 +509,7 @@ void demo_rbtree_struct(void)
     printf("\n");
 
     printf("----- inorder -----\n");
-    iter = tree->iter(tree, ORDER_IN);
+    iter = tree->iter(tree, TREE_DFS_IN);
     while(iter->hasnext(iter))
     {
         temp = *(struct _student *)iter->next(iter);
@@ -518,7 +518,7 @@ void demo_rbtree_struct(void)
     printf("\n");
 
     printf("----- postorder -----\n");
-    iter = tree->iter(tree, ORDER_POST);
+    iter = tree->iter(tree, TREE_DFS_POST);
     while(iter->hasnext(iter))
     {
         temp = *(struct _student *)iter->next(iter);
@@ -527,7 +527,7 @@ void demo_rbtree_struct(void)
     printf("\n");
 
     printf("----- breadth -----\n");
-    iter = tree->iter(tree, ORDER_BREADTH);
+    iter = tree->iter(tree, TREE_BFS);
     while(iter->hasnext(iter))
     {
         temp = *(struct _student *)iter->next(iter);
@@ -536,7 +536,7 @@ void demo_rbtree_struct(void)
     printf("\n");
 
     printf("----- preorder(right) -----\n");
-    iter = tree->iter(tree, ORDER_PRE_R);
+    iter = tree->iter(tree, TREE_DFS_PRE_R);
     while(iter->hasnext(iter))
     {
         temp = *(struct _student *)iter->next(iter);
@@ -545,7 +545,7 @@ void demo_rbtree_struct(void)
     printf("\n");
 
     printf("----- inorder(right) -----\n");
-    iter = tree->iter(tree, ORDER_IN_R);
+    iter = tree->iter(tree, TREE_DFS_IN_R);
     while(iter->hasnext(iter))
     {
         temp = *(struct _student *)iter->next(iter);
@@ -554,7 +554,7 @@ void demo_rbtree_struct(void)
     printf("\n");
 
     printf("----- postorder(right) -----\n");
-    iter = tree->iter(tree, ORDER_POST_R);
+    iter = tree->iter(tree, TREE_DFS_POST_R);
     while(iter->hasnext(iter))
     {
         temp = *(struct _student *)iter->next(iter);
@@ -563,7 +563,7 @@ void demo_rbtree_struct(void)
     printf("\n");
 
     printf("----- breadth(right) -----\n");
-    iter = tree->iter(tree, ORDER_BREADTH_R);
+    iter = tree->iter(tree, TREE_BFS_R);
     while(iter->hasnext(iter))
     {
         temp = *(struct _student *)iter->next(iter);
@@ -572,7 +572,7 @@ void demo_rbtree_struct(void)
     printf("\n");
 
     printf("----- preorder -----\n");
-    iter = tree->iter(tree, ORDER_PRE);
+    iter = tree->iter(tree, TREE_DFS_PRE);
     while(iter->hasnext(iter))
     {
         temp = *(struct _student *)iter->next(iter);
@@ -591,7 +591,7 @@ void demo_rbtree_struct(void)
 
         printf("size = %2d : ", tree->size(tree));
 
-        iter = tree->iter(tree, ORDER_PRE);
+        iter = tree->iter(tree, TREE_DFS_PRE);
         while(iter->hasnext(iter))
         {
             temp = *(struct _student *)iter->next(iter);
diff --git a/include/tree.h b/include/tree.h
index 7353187..4d8b82c 100644
--- a/include/tree.h
+++ b/include/tree.h
@@ -16,23 +16,22 @@
 #include "stack.h"
 #include "queue.h"
 
-
-typedef enum {
+typedef enum _rbt_color{
     RBT_RED,
     RBT_BLACK,
 }rbt_color;
 
 // dfs and bfs traversal order
 enum _tree_order{
-    ORDER_PRE,      // pre-order
-    ORDER_IN,       // in-order
-    ORDER_POST,     // post-order
-    ORDER_BREADTH,  // breadth-first search [BFS]
+    TREE_DFS_PRE,       // pre-order
+    TREE_DFS_IN,        // in-order
+    TREE_DFS_POST,      // post-order
+    TREE_BFS,           // breadth-first search [BFS]
 
-    ORDER_PRE_R,    // right-first pre-order
-    ORDER_IN_R,     // right-first in-order
-    ORDER_POST_R,   // right-first post-order
-    ORDER_BREADTH_R,// right-first breadth-first search [BFS]
+    TREE_DFS_PRE_R,     // right-first pre-order
+    TREE_DFS_IN_R,      // right-first in-order
+    TREE_DFS_POST_R,    // right-first post-order
+    TREE_BFS_R,         // right-first breadth-first search [BFS]
 };
 
 struct _tree_node
@@ -45,7 +44,7 @@ struct _tree_node
     union 
     {
         int32_t balance;
-        uint32_t color;
+        rbt_color color;
     };
 };
 
diff --git a/src/map.c b/src/map.c
index e2e1343..c2b2cf5 100644
--- a/src/map.c
+++ b/src/map.c
@@ -58,7 +58,7 @@ static void map_node_free(struct _map_node** node)
 bool map_clear(struct _map* self)
 {
     struct _map_node* node = NULL;
-    iterator_t iter = self->_tree->iter(self->_tree, ORDER_IN);
+    iterator_t iter = self->_tree->iter(self->_tree, TREE_DFS_IN);
     while(iter->hasnext(&iter))
     {
         // tree & map malloc memory independently, so free them one by one.
diff --git a/src/tree.c b/src/tree.c
index 9691fcf..b440aa5 100644
--- a/src/tree.c
+++ b/src/tree.c
@@ -12,8 +12,7 @@
 #include "queue.h"
 #include "stack.h"
 
-// #define TREE_RECURSIVE_ENABLED 
-
+// #define TREE_RECURSIVE_ENABLED
 static struct _tree_node* tree_node_new(struct _tree* self, void* obj)
 {
     assert(self != NULL);
@@ -1151,11 +1150,11 @@ static const void* tree_iter_next(struct _iterator* iter)
     struct _tree_node* target_node = NULL;
     switch (iter->_order)
     {
-    case ORDER_PRE:
-    case ORDER_PRE_R:
+    case TREE_DFS_PRE:
+    case TREE_DFS_PRE_R:
     {
         struct _tree_node* node = NULL;
-        if (iter->_order == ORDER_PRE)
+        if (iter->_order == TREE_DFS_PRE)
         {
             while (!self->stack->empty(self->stack) || cur_node != NULL)
             {
@@ -1194,10 +1193,10 @@ static const void* tree_iter_next(struct _iterator* iter)
             }
         }
     }break;
-    case ORDER_IN:
-    case ORDER_IN_R:
+    case TREE_DFS_IN:
+    case TREE_DFS_IN_R:
     {
-        if (iter->_order == ORDER_IN)
+        if (iter->_order == TREE_DFS_IN)
         {
             while (!self->stack->empty(self->stack) || cur_node != NULL)
             {
@@ -1236,8 +1235,8 @@ static const void* tree_iter_next(struct _iterator* iter)
             }
         }
     }break;
-    case ORDER_POST:
-    case ORDER_POST_R:
+    case TREE_DFS_POST:
+    case TREE_DFS_POST_R:
     {
         if (!self->stack->empty(self->stack))
         {
@@ -1245,8 +1244,8 @@ static const void* tree_iter_next(struct _iterator* iter)
             target_node = cur_node;
         }
     }break;
-    case ORDER_BREADTH:
-    case ORDER_BREADTH_R:
+    case TREE_BFS:
+    case TREE_BFS_R:
     {
         queue_t queue = self->queue;
         if (!queue->empty(queue) && cur_node != NULL)
@@ -1254,7 +1253,7 @@ static const void* tree_iter_next(struct _iterator* iter)
             queue->pop(queue, &cur_node);
             target_node = cur_node;
 
-            if (iter->_order == ORDER_BREADTH)
+            if (iter->_order == TREE_BFS)
             {
                 if (cur_node->left != NULL)
                 {
@@ -1307,24 +1306,24 @@ static iterator_t tree_iter(struct _tree* self, enum _tree_order order)
 
     switch (iter->_order)
     {
-    case ORDER_PRE:
-    case ORDER_PRE_R:
+    case TREE_DFS_PRE:
+    case TREE_DFS_PRE_R:
     {
         // pass
     }break;
-    case ORDER_IN:
-    case ORDER_IN_R:
+    case TREE_DFS_IN:
+    case TREE_DFS_IN_R:
     {
         // pass
     }break;
-    case ORDER_POST:
-    case ORDER_POST_R:
+    case TREE_DFS_POST:
+    case TREE_DFS_POST_R:
     {
         struct _tree_node* node = self->_root;
         self->stack->clear(self->stack);
 
         stack_t stack = stack_new(sizeof(struct _tree_node*));
-        if (iter->_order == ORDER_POST)
+        if (iter->_order == TREE_DFS_POST)
         {
             while (!stack->empty(stack) || node != NULL)
             {
@@ -1362,8 +1361,8 @@ static iterator_t tree_iter(struct _tree* self, enum _tree_order order)
         }
         stack_free(&stack);
     }break;
-    case ORDER_BREADTH:
-    case ORDER_BREADTH_R:
+    case TREE_BFS:
+    case TREE_BFS_R:
     {
         // pass
         self->queue->push(self->queue, &self->_root);
diff --git a/test/test_tree.c b/test/test_tree.c
index f64a0c4..fe3feae 100644
--- a/test/test_tree.c
+++ b/test/test_tree.c
@@ -68,8 +68,8 @@ static const int expected_int_array_orderpre_delete[15][15] = {
 };
 
 static const enum _tree_order order[8] = { 
-    ORDER_PRE, ORDER_IN, ORDER_POST, ORDER_BREADTH,
-    ORDER_PRE_R, ORDER_IN_R, ORDER_POST_R, ORDER_BREADTH_R
+    TREE_DFS_PRE, TREE_DFS_IN, TREE_DFS_POST, TREE_BFS,
+    TREE_DFS_PRE_R, TREE_DFS_IN_R, TREE_DFS_POST_R, TREE_BFS_R
 };
 
 static uint32_t iter2array_num(iterator_t iter, int *data)
@@ -104,7 +104,7 @@ static void test_avltree_iter(void)
         temp = data[i];
         TEST_ASSERT_TRUE(tree->insert(tree, &temp));
 
-        iter = tree->iter(tree, ORDER_PRE);
+        iter = tree->iter(tree, TREE_DFS_PRE);
         count = iter2array_num(iter, buff);
         TEST_ASSERT_EQUAL_INT_ARRAY(expected_int_array_orderpre_insert[i], buff, count);
     }
@@ -145,7 +145,7 @@ static void test_avltree_insert(void)
         temp = data[i];
         TEST_ASSERT_TRUE(tree->insert(tree, &temp));
 
-        iter = tree->iter(tree, ORDER_PRE);
+        iter = tree->iter(tree, TREE_DFS_PRE);
         count = iter2array_num(iter, buff);
         TEST_ASSERT_EQUAL_INT_ARRAY(expected_int_array_orderpre_insert[i], buff, count);
     }
@@ -185,7 +185,7 @@ static void test_avltree_delete(void)
 
     for (i = 0; i < len; i++)
     {
-        iter = tree->iter(tree, ORDER_PRE);
+        iter = tree->iter(tree, TREE_DFS_PRE);
         count = iter2array_num(iter, buff);
         TEST_ASSERT_EQUAL_INT_ARRAY(expected_int_array_orderpre_delete[i], buff, count);
 
@@ -270,7 +270,7 @@ static void test_rbtree_iter(void)
         temp = data[i];
         TEST_ASSERT_TRUE(tree->insert(tree, &temp));
         
-        iter = tree->iter(tree, ORDER_PRE);
+        iter = tree->iter(tree, TREE_DFS_PRE);
         count = iter2array_num(iter, buff);
         TEST_ASSERT_EQUAL_INT_ARRAY(rbt_expected_int_array_orderpre_insert[i], buff, count);
     }
@@ -310,7 +310,7 @@ static void test_rbtree_insert(void)
         temp = data[i];
         TEST_ASSERT_TRUE(tree->insert(tree, &temp));
 
-        iter = tree->iter(tree, ORDER_PRE);
+        iter = tree->iter(tree, TREE_DFS_PRE);
         count = iter2array_num(iter, buff);
         TEST_ASSERT_EQUAL_INT_ARRAY(rbt_expected_int_array_orderpre_insert[i], buff, count);
     }
@@ -349,7 +349,7 @@ static void test_rbtree_delete(void)
 
     for (i = 0; i < len; i++)
     {
-        iter = tree->iter(tree, ORDER_PRE);
+        iter = tree->iter(tree, TREE_DFS_PRE);
         count = iter2array_num(iter, buff);
         TEST_ASSERT_EQUAL_INT_ARRAY(rbt_expected_int_array_orderpre_delete[i], buff, count);