C :

#include <stdio.h>
#include <string.h>
void build(char *pre,char *in,int len){
	if (len<=0)
	{
		return ;
	}
	int pos=strchr(in,*pre)-in;
	build(pre+1,in,pos);
	build(pre+pos+1,in+pos+1,len-pos-1);
	printf("%c",*pre);

}
int main(){
	char pre[1000],in[1000];
//	freopen("1.txt","r",stdin);
	while (scanf("%s %s",pre,in)!=EOF)
	{
		build(pre,in,strlen(pre));
		printf("\n");
	}
//	fclose(stdin);
	return 0;
}

C++ :

#include<cstdio>
#include<iostream>
#include<string>
using namespace std;

struct BT
{
	char data;
	BT *lchild,*rchild;
};

BT *build(string preorder,string inorder)
{
	BT *root;
	int p;
	if(!preorder.length())
		root=NULL;
	else
	{
		root=new BT;
		root->data=preorder[0];
		p=inorder.find(root->data);
		root->lchild=build(preorder.substr(1,p),inorder.substr(0,p));
		root->rchild=build(preorder.substr(1+p),inorder.substr(p+1));
	}
	return root;
}

void postorder(BT *root)
{
	if(root)
	{
		postorder(root->lchild);
		postorder(root->rchild);
		putchar(root->data);
	}
}

int main()
{
	string preorder,inorder;
	BT *root;
	while(cin>>preorder>>inorder)
	{
		root=build(preorder,inorder);
		postorder(root);
		printf("\n");
	}
	return 0;
}

Java :

import java.util.Scanner;

public class Main {
	public static void main(String args[]) {
		Scanner jin = new Scanner(System.in);
		while (jin.hasNext()) {
			String pre = jin.next(), mid = jin.next();
			Node root = new Node(pre);
			root.build(pre, mid);
			root.post();
			System.out.println();
		}
	}
}

class Node {
	char entry;
	Node lson;
	Node rson;

	public Node(String pre) {
		entry = pre.charAt(0);
		lson = null;
		rson = null;
	}

	public void build(String pre, String mid) {
		if (pre.length() > 1) {
			int p = mid.indexOf(entry);
			String lmid = mid.substring(0, p);
			String rmid = mid.substring(p + 1);
			String lpre = pre.substring(1, 1 + lmid.length());
			String rpre = pre.substring(1 + lmid.length());
			if (lpre.length() > 0) {
				lson = new Node(lpre);
				lson.build(lpre, lmid);
			}
			if (rpre.length() > 0) {
				rson = new Node(rpre);
				rson.build(rpre, rmid);
			}
		}
	}

	public void post() {
		if (lson != null)
			lson.post();
		if (rson != null)
			rson.post();
		System.out.print(entry);
	}
}