Fix the test case of the binary tree dfs in Java.

codes/java/chapter_tree/binary_tree_dfs.java

@@ -43,7 +43,7 @@ public class binary_tree_dfs {
     public static void main(String[] args) {
         /* 初始化二叉树 */
         // 这里借助了一个从数组直接生成二叉树的函数
-        TreeNode root = TreeNode.arrToTree(new Integer[] { 1, null, 3, 4, 5 });
+        TreeNode root = TreeNode.arrToTree(new Integer[] { 1, 2, 3, 4, 5, 6, 7 });
         System.out.println("\n初始化二叉树\n");
         PrintUtil.printTree(root);