AC

#include<bits/stdc++.h> 
using namespace std;
string s1,s2;
void maketree(int left1,int right1,int left2,int right2){		//1是先序遍历，2是中序遍历。left是左端点，right是右端点 
	char root=s1[left1];	//left1~right1为先序序列，left2~right2为中序序列 
	int find;
	for(find=left2;find<=right2;find++){		//在中序遍历中找到根节点 
		if(s2[find]==root){
			maketree(left1+1, left1+find-left2, left2, find-1);		//左子树 
			maketree(right1-(right2-find-1), right1, find+1, right2);		//右子树 
			cout<<root;
		}
	}
} 
int main(){
	cin>>s1>>s2;
	maketree(0,s1.size()-1,0,s2.size()-1);
	return 0;
}