Want to save binary tree to disk for “20 questions” game

Question

In short, I\'d like to learn/develop an elegant method to save a binary tree to disk (a general tree, not necessarily a BST). Here is the description of my problem:

I\'

Answer 1

Here is the C++ code using PreOrder DFS:

void SaveBinaryTreeToStream(TreeNode* root, ostringstream& oss)
{
    if (!root)
    {
        oss << '#';
        return;
    }

    oss << root->data;
    SaveBinaryTreeToStream(root->left, oss);
    SaveBinaryTreeToStream(root->right, oss);
}
TreeNode* LoadBinaryTreeFromStream(istringstream& iss)
{
    if (iss.eof())
        return NULL;

    char c;
    if ('#' == (c = iss.get()))
        return NULL;

    TreeNode* root = new TreeNode(c, NULL, NULL);
    root->left  = LoadBinaryTreeFromStream(iss);
    root->right = LoadBinaryTreeFromStream(iss);

    return root;
}

In main(), you can do:

ostringstream oss;
root = MakeCharTree();
PrintVTree(root);
SaveBinaryTreeToStream(root, oss);
ClearTree(root);
cout << oss.str() << endl;
istringstream iss(oss.str());
cout << iss.str() << endl;
root = LoadBinaryTreeFromStream(iss);
PrintVTree(root);
ClearTree(root);

/* Output:
               A

       B               C

   D               E       F

     G           H   I
ABD#G###CEH##I##F##
ABD#G###CEH##I##F##
               A

       B               C

   D               E       F

     G           H   I
 */

The DFS is easier to understand.

*********************************************************************************

But we can use level scan BFS using a queue

ostringstream SaveBinaryTreeToStream_BFS(TreeNode* root)
{
    ostringstream oss;

    if (!root)
        return oss;

    queue q;
    q.push(root);

    while (!q.empty())
    {
        TreeNode* tn = q.front(); q.pop();

        if (tn)
        {
            q.push(tn->left);
            q.push(tn->right);
            oss << tn->data;
        }
        else
        {
            oss << '#';
        }
    }

    return oss;
}
TreeNode* LoadBinaryTreeFromStream_BFS(istringstream& iss)
{
    if (iss.eof())
        return NULL;

    TreeNode* root = new TreeNode(iss.get(), NULL, NULL);
    queue q; q.push(root); // The parents from upper level
    while (!iss.eof() && !q.empty())
    {
        TreeNode* tn = q.front(); q.pop();

        char c = iss.get();
        if ('#' == c)
            tn->left = NULL;
        else
            q.push(tn->left = new TreeNode(c, NULL, NULL));

        c = iss.get();
        if ('#' == c)
            tn->right = NULL;
        else
            q.push(tn->right = new TreeNode(c, NULL, NULL));
    }

    return root;
}

In main(), you can do:

root = MakeCharTree();
PrintVTree(root);
ostringstream oss = SaveBinaryTreeToStream_BFS(root);
ClearTree(root);
cout << oss.str() << endl;
istringstream iss(oss.str());
cout << iss.str() << endl;
root = LoadBinaryTreeFromStream_BFS(iss);
PrintVTree(root);
ClearTree(root);

/* Output:
               A

       B               C

   D               E       F

     G           H   I
ABCD#EF#GHI########
ABCD#EF#GHI########
               A

       B               C

   D               E       F

     G           H   I
 */