POJ2255 Tree Recovery 【樹的遍曆】

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Tree Recovery

Time Limit: 1000MS		Memory Limit: 65536K
Total Submissions: 11365		Accepted: 7128

Description

Little Valentine liked playing with binary trees very much. Her favorite game was constructing randomly looking binary trees with capital letters in the nodes. 
This is an example of one of her creations: 

                                               D                                              /                                              /                                               B     E                                           / \                                               /   \                                              A     C     G                                                    /                                                   /                                                  F

To record her trees for future generations, she wrote down two strings for each tree: a preorder traversal (root, left subtree, right subtree) and an inorder traversal (left subtree, root, right subtree). For the tree drawn above the preorder traversal is DBACEGF and the inorder traversal is ABCDEFG. 
She thought that such a pair of strings would give enough information to reconstruct the tree later (but she never tried it). 

Now, years later, looking again at the strings, she realized that reconstructing the trees was indeed possible, but only because she never had used the same letter twice in the same tree. 
However, doing the reconstruction by hand, soon turned out to be tedious. 
So now she asks you to write a program that does the job for her! 

Input

The input will contain one or more test cases. 
Each test case consists of one line containing two strings preord and inord, representing the preorder traversal and inorder traversal of a binary tree. Both strings consist of unique capital letters. (Thus they are not longer than 26 characters.) 
Input is terminated by end of file. 

Output

For each test case, recover Valentine‘s binary tree and print one line containing the tree‘s postorder traversal (left subtree, right subtree, root).

Sample Input

DBACEGF ABCDEFGBCAD CBAD

Sample Output

ACBFGEDCDAB

前序走訪的第一個點必定是根節點，在對應的中序遍曆中找到這個點，那麼在中序序列該點左邊的子序列必定是左子樹，右邊是右子樹，於是就這樣遞迴下去，便能找到後序序列。

#include <stdio.h>#include <string.h>#define maxn 28char str1[maxn], str2[maxn];void traverse(int l1, int r1, int l2, int r2){if(l1 > r1) return;int rt;rt = strchr(str2, str1[l1]) - str2;traverse(l1 + 1, rt - l2 + l1, l2, rt - 1);traverse(rt-l2+l1+1, r1, rt+1, r2);putchar(str2[rt]);}int main(){int len;while(scanf("%s%s", str1, str2) == 2){len = strlen(str1);traverse(0, len - 1, 0, len - 1);printf("\n");}return 0;}