Tree Recovery--POJ 2255

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

struct Node{
    char data;
    Node* lchild;
    Node* rchild;
};

string prestr,midstr;

Node* CreateTree(string pre,string mid)
{
    Node* root=NULL;
    if(pre.length()>0)
    {
        root=new Node;
        root->data=pre[0];
        int index=mid.find(root->data);
        root->lchild=CreateTree(pre.substr(1,index),mid.substr(0,index));
        root->rchild=CreateTree(pre.substr(index+1),mid.substr(index+1));
    }
    return root;
}

void PostOrder(Node* root)
{
    if(root!=NULL)
    {
        PostOrder(root->lchild);
        PostOrder(root->rchild);
        cout<<root->data;
    }
}
int main()
{
    while(cin>>prestr>>midstr)
    {
        Node* tmp=CreateTree(prestr,midstr);
        PostOrder(tmp);
        cout<<endl;
    }
    return 1;
}