The key of this problem is that we need not build the tree from scratch. In fact, we can direct obtain its post-order traversal results in a recursive manner and the problem has given nice hints on this.

The code is as follows.

1 #include 
     
       2 #include 
      
        3  4 using namespace std; 5  6 string post_order(string pre_order, string in_order) { 7     if (pre_order.empty() && in_order.empty()) 8         return ""; 9     if (pre_order.length() == 1 && in_order.length() == 1)10         return pre_order;11     string root = pre_order.substr(0, 1);12     int rootInOrder = 0;13     while (in_order[rootInOrder] != pre_order[0])14         rootInOrder++;15     string pl = pre_order.substr(1, rootInOrder);16     string pr = pre_order.substr(1 + pl.length(), pre_order.length() - pl.length() - 1);17     string il = in_order.substr(0, rootInOrder);18     string ir = in_order.substr(rootInOrder + 1, in_order.length() - il.length() - 1);19     return post_order(pl, il) + post_order(pr, ir) + root;20 }21 22 int main(void) {23     string pre_order, in_order;24     while (cin >> pre_order >> in_order)25         cout << post_order(pre_order, in_order);26     return 0;27 }