从空二开始，严格按照二叉搜索树的插入算法（不进行旋转平衡），逐个插入关键码{18,73,10,5,68,99,27,41,51,32,25}构造出一棵二叉搜索树，对该二叉搜索树按照前序遍历得到的序列为？（答案中每两个元素之间用一个空格隔开）From a null binary tree, insert key values {18, 73, 10, 5, 68, 99, 27, 41, 51, 32, 25} successively according to the insertion algorithm of a binary search tree strictly (no rotation and balance) to construct a binary search tree. Please write down the sequence of preorder of this binary search tree. (There is one blank space between two elements)