树的打印函数好像对null处理有点问题，麻烦大佬帮看下
Author: 1580923067Created Aug 8, 2020Updated Mar 27, 2024
例如Leetcode94题，输入: [1,null,2,3]，调用TreeNode.print( )函数，3会丢失
Source: Blankj/awesome-java-leetcode
例如Leetcode94题，输入: [1,null,2,3]，调用TreeNode.print( )函数，3会丢失
Source: Blankj/awesome-java-leetcode