说明：  假设树上第个结点所含的数据元素为一个字母，并用链式存储结构为树的存储结构，试写一个按凹入表达方式打印一棵树的算法
(Assumes that a tree each node has data elements of a letter, and chain storage structure for the storage structure of tree, try to write a press recessed expression print a tree algorithm)