diff --git a/codes/cpp/chapter_tree/binary_tree_bfs.cpp b/codes/cpp/chapter_tree/binary_tree_bfs.cpp
index ce0072b..eeec2a1 100644
--- a/codes/cpp/chapter_tree/binary_tree_bfs.cpp
+++ b/codes/cpp/chapter_tree/binary_tree_bfs.cpp
@@ -17,9 +17,9 @@ vector<int> hierOrder(TreeNode* root) {
         TreeNode* node = queue.front();
         queue.pop();  // 队列出队
         vec.push_back(node->val);            // 保存结点
-        if (node->left != NULL)
+        if (node->left != nullptr)
             queue.push(node->left);    // 左子结点入队
-        if (node->right != NULL)
+        if (node->right != nullptr)
             queue.push(node->right);   // 右子结点入队
     }
     return vec;
diff --git a/docs/chapter_tree/binary_tree.md b/docs/chapter_tree/binary_tree.md
index 422b259..58034b1 100644
--- a/docs/chapter_tree/binary_tree.md
+++ b/docs/chapter_tree/binary_tree.md
@@ -238,9 +238,9 @@ comments: true
             TreeNode* node = queue.front();
             queue.pop();  // 队列出队
             vec.push_back(node->val);            // 保存结点
-            if (node->left != NULL)
+            if (node->left != nullptr)
                 queue.push(node->left);    // 左子结点入队
-            if (node->right != NULL)
+            if (node->right != nullptr)
                 queue.push(node->right);   // 右子结点入队
         }
         return vec;