C :

#include<stdio.h>
#include<string.h>
char a[10],b[10],s[10];
int i;
void f(int n,char *a,char *b,char *s)
{
    int p;
    if(n<=0)return ;
    p=strchr(a,b[n-1])-a;
    s[i++]=b[n-1];
    f(p,a,b,s);
    f(n-p-1,a+p+1,b+p,s);

}
int main()
{
    int n;
    gets(a); gets(b);
    n=strlen(a);
    f(n,a,b,s);
    s[i]=0;
    puts(s);
    return 0;
}

C++ :

#include<iostream>
#include<algorithm>
#include<cstring>
#include<ctime>
#include<cmath>
#include<string>
#include<cstdio>
using namespace std;

char a[15], b[15];
int v[15], pos;

struct tree{
    char ch;
    struct tree *lson, *rson;
}*root;

bool right(char ch)
{
    int i;
    for(i = 0; a[i]; ++i)
    {
        if(a[i] == ch) {
            v[i] = true; break;
        }
    }
    i++;
    if(!v[i] && a[i] != '\0') return true;
    return false;
}

bool left(char ch)
{
    int i;
    for(i = 0; a[i]; ++i)
    {
        if(a[i] == ch) {
            v[i] = true; break;
        }
    }
    i--;
    if(i >= 0 && !v[i]) return true;
    return false;
}

void build(struct tree *rt)
{
    //cout<<pos<<endl;
    rt->ch = b[pos--];
    rt->lson = new tree;
    rt->rson = new tree;
    if( right(rt->ch) )
        build(rt->rson);
    else rt->rson = NULL;
    if( left(rt->ch) )
        build(rt->lson);
    else rt->lson = NULL;
}

void travel(struct tree *rt)
{
    if(rt == NULL) return;
    putchar(rt->ch);
    travel(rt->lson);
    travel(rt->rson);
}

int main()
{
    cin>>a>>b;
    memset(v, false, sizeof(v));
    root = new tree;
    pos = strlen(b) - 1;
    build(root);
    travel(root); puts("");
    return 0;
}