层次遍历多元树(在文件tree.cpp中3个空)、先根遍历、后根遍历的递归函数
Hierarchical traversal of multivariate trees (3 empty in the file tree.cpp), first root traversal, post root traversal recursive function
Hierarchical traversal of multivariate trees (3 empty in the file tree.cpp), first root traversal, post root traversal recursive function