#include <string>
#include <iostream>
#include <iomanip>

struct node
{ string data;
  node * left, * right;
  node(string s); };

node::node(string s)
{ data = s;
  left = right = NULL; }

struct tree
{ node * root;
  tree();
  void insert(string s);
  void insert(string s, node * & ptr);
  void print();
  void print(node * ptr, int depth); };

tree::tree()
{ root = NULL; }

void tree::insert(string s)
{ insert(s, root); }

void tree::insert(string s, node * & ptr)
{ if (ptr == NULL)
    ptr = new node(s);
  else if (s < ptr->data)
    insert(s, ptr->left);
  else
    insert(s, ptr->right); }
 
void tree::print()
{ print(root, 0); }

void tree::print(node * ptr, int depth)   // indentation shows the tree's structure
{ if (ptr != NULL)
  { print(ptr->left, depth+1);
    cout << setw(depth*3) << "" << ptr->data << "\n";
    print(ptr->right, depth+1); } }

void main()
{ tree T;
  while (true)
  { T.print();
    cout << "? ";
    string s;
    cin >> s;
    cout << "\n";
    if (cin.fail())
      break;
    T.insert(s); }
  T.print(); }