多元树的层次遍历、先根遍历、后根遍历 涉及的知识点：多元树的表示法、先根遍历、后根遍历的递归算法、多元树的层次遍历方法-Multi-level tree traversal, first the root traversal, traversal involved in dorsal root knowledge point: multi-tree representation, first the root traversal, after the root of the recur