Imagine a scenario in which you want to traverse a directory tree recursively, stopping at each file and directory under that tree and having your own callback methods invoked for each entry found.
设想一个场景，您想要递归地访问一个目录树，在该树中的每个文件及目录上停下来，并为每个查找到的条目调用您自己的回调方法。
In this step, you will traverse the source file system tree.
在这个步骤中，您将遍历源文件系统树。
However, three methods also need to traverse each file, similar methods 2 and relatively time-consuming.
然而方法3也需要遍历每个文件，因此类似方法2，也比较耗时。
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