Fix the util of array to tree.

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;

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()