实现树的建立，前序中须后续三种遍历，计算总节点数，叶子节点数，树深度等-To achieve the establishment of the tree, the preamble to be the subsequent three kinds of traversal, calculating the total number of nodes, leaf nodes, the tree depth

数据结构难点，树，实现树的前序中序后序遍历，对初学数据结构很有启发-Data structure difficulty, tree, tree realize after preorder preamble sequence, the data structure is very enlightening for beginners