思路

1.前序遍历：root [左子树] [右子树]
2.中序遍历：[左子树] root [右子树]
3.递归，找到每次的子树边界就行。

code

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

typedef struct Node{
    char data;
    Node * lc;
    Node * rc; 
}Node;

const int N = 30;

string pre;
string mid;

Node * buildTree(int pl, int pr, int ml, int mr){
    if(pl > pr){ // 递归边界
        return NULL;
    }
    Node* p = new Node();

    // cout << p->data << " " ;    

    int pos = mid.find(pre.at(pl));
    int sz = pos - ml;
    p->lc = buildTree(pl+1, pl + sz, ml, pos-1);// 左子树      
    p->rc = buildTree(pl+sz+1, pr, pos+1, mr); // 右子树
    p->data = pre.at(pl); // 树根
    cout << p->data;
    return p;
}

int main(){
    while(cin >> pre >> mid){
        Node * tree = buildTree(0,pre.length()-1,0,mid.length()-1);
        cout << endl;
    }
    system("pause");
    return 0;
}