题目只是要求输出后序遍历，这样就省事多了。无需建树，用数组来表示每个节点的字母，递归输出后序遍历即可。

/*************************************************************************
      > File Name: Tree Recovery UVA - 536
      > Author: Mrhanice
      > Mail: 690697134@qq.com
      > Created Time: 20170312
      > link : https://vjudge.net/problem/UVA-536
  ************************************************************************/
#include <cstdio>
#include <cstring>
using namespace std;

const int maxn=30;
char in_order[maxn],post_order[maxn],pre_order[maxn];

void post_trva(char *pre, char *in ,int length)
{
    if(length<=0) return ;
    char x = *pre;
    int i=0;
    for(;i<length;i++)
    {
        if(in[i]==x)
            break;
    }
    post_trva(pre+1,in,i);
    post_trva(pre+i+1,in+i+1,length-i-1);
    printf("%c",x);
}
int main()
{
    while(scanf("%s",pre_order)!=EOF)
    {
        scanf("%s",in_order);
        int len=strlen(pre_order);
        post_trva(pre_order,in_order,len);
        printf("\n");
    }
    return 0;
}