二叉树遍历(Traversal)是指沿着某条搜索路线，依次对树中每个结点均做一次且仅做一次访问。访问结点所做的操作依赖于具体的应用问 题。-Binary tree Traversal (Traversal) refers to the search along a route, in turn, each node in the tree are one and only for a visit. Operations to do access nodes depends on the spec