未经整理的题解：

#include <bits/stdc++.h>
using namespace std;
char a[35], b[35];
void dfs(int aL, int aR, int bL, int bR)
{
    if (aL > aR) return;
    int p = 0;
    for (int j = bL; j <= bR; j++)
    {
        for (int i = aL; i <= aR; i++) if (b[j] == a[i]) { p = i; break; }
        if (p) break;
    }
    printf("%c", a[p]);
    dfs(aL, p - 1, bL, bR);
    dfs(p + 1, aR, bL, bR);
    
}
int main()
{
    scanf("%s%s", a + 1, b + 1);
    int len = strlen(a + 1);
    dfs(1, len, 1, len);
    return 0;
}

优化后的代码：

#include <bits/stdc++.h>
using namespace std;
char a[35], b[35], ys[150];
void dfs(int aL, int aR, int bL, int bR)
{
    if (aL > aR) return;
    int p = 0; for (int j = bL; j <= bR; j++) if (ys[b[j]] >= aL && ys[b[j]] <= aR) { p = ys[b[j]]; break; }
    printf("%c", a[p]);
    dfs(aL, p - 1, bL, bR);
    dfs(p + 1, aR, bL, bR);
    
}
int main()
{
    scanf("%s%s", a + 1, b + 1);
    int len = strlen(a + 1);
    for (int i = 1; i <= len; i++) ys[a[i]] = i;
    dfs(1, len, 1, len);
    return 0;
}