掌握二叉树的创建、遍历的方法。 利用二叉树的按层遍历序列创建二叉树，然后实现二叉树的前序、中序和后序遍历。-Master binary tree creation, traversal methods. The use of binary tree traversal sequences by layer to create a binary tree, and then to achieve binary tree preorder, inorder and postorder trave