diff --git a/README.md b/README.md
index 24391cf..a006289 100644
--- a/README.md
+++ b/README.md
@@ -29,7 +29,7 @@
 
 ## 关于本书
 
-本书致力于达成以下目标：
+本书面向数据结构与算法初学者，致力于达成以下目标：
 
 - 开源免费，所有同学都可在网上获取本书；
 - 新手友好，适合算法初学者自主学习入门；
diff --git a/codes/java/include/TreeNode.java b/codes/java/include/TreeNode.java
index bba76b1..6541064 100755
--- a/codes/java/include/TreeNode.java
+++ b/codes/java/include/TreeNode.java
@@ -26,6 +26,9 @@ public class TreeNode {
      * @return
      */
     public static TreeNode arrToTree(Integer[] arr) {
+        if (arr.length == 0)
+            return null;
+        
         TreeNode root = new TreeNode(arr[0]);
         Queue<TreeNode> queue = new LinkedList<>() {{ add(root); }};
         int i = 1;
diff --git a/codes/python/include/binary_tree.py b/codes/python/include/binary_tree.py
index d905d82..24acb47 100644
--- a/codes/python/include/binary_tree.py
+++ b/codes/python/include/binary_tree.py
@@ -24,7 +24,7 @@ def list_to_tree(arr):
         [type]: [description]
     """
     if not arr:
-        return
+        return None
     i = 1
     root = TreeNode(int(arr[0]))
     queue = collections.deque()