二叉树的双序遍历（对于二叉树的每一个结点来说，先访问这个结点，再按双序遍历它的左子树，然后再一次访问这个结点，接下来按双序遍历它的右子树）算法-Dual binary tree traversal (for binary tree each node, the first access to this node, and then double traversal of its left subtree, then visit this node, then double-click it in