### 1127. ZigZagging on a Tree (30)

Suppose that all the keys in a binary tree are distinct positive integers. A unique binary tree can be determined by a given pair of postorder and inorder traversal sequences. And it is a simple standard routine to print the numbers in level-order. However, if you think the problem is too simple, then you are too naive. This time you are supposed to print the numbers in “zigzagging order” – that is, starting from the root, print the numbers level-by-level, alternating between left to right and right to left. For example, for the following tree you must output: 1 11 5 8 17 12 20 15.

Input Specification:

Each input file contains one test case. For each case, the first line gives a positive integer N (<= 30), the total number of nodes in the binary tree. The second line gives the inorder sequence and the third line gives the postorder sequence. All the numbers in a line are separated by a space.

Output Specification:

For each test case, print the zigzagging sequence of the tree in a line. All the numbers in a line must be separated by exactly one space, and there must be no extra space at the end of the line.

Sample Input:
8
12 11 20 17 1 15 8 5
12 20 17 11 15 8 5 1
Sample Output:
1 11 5 8 17 12 20 15

### 解题报告

1. 建树
1. 找出后序中最后一个元素在中序的位置，由这个位置将中序和后序一分为二，即两个序列左右部分元素个数相同。
2. 以后序最后一个元素为根，将分开的两部分依次按同样方法建立左右子树。
2. 输出，区分每一层的元素个数，判断向左还是向右，存入栈中，当一层进行完，再由栈进入队列中，保证顺序正确。