How to display binary search tree in console properly?

Question

I'm trying to display a BST in console. That's my code (it's a modified version of code found here: Printing Level Order Binary Search Tree Formatting):

string printLevel(node *root, int level, string gap) {
  if (!root) {
    return gap + "-" + gap;
  }
  if (level==1) {
    stringstream out;
    out<<root->value;
    return gap + out.str() + gap;
  } else if (level>1) {
    string leftStr = printLevel(root->left, level-1, gap);
    string rightStr = printLevel(root->right, level-1, gap);
    return leftStr + " " + rightStr;
  } else return "";
}

void printLevelOrder (node* root, int depth) {
  for (int i=1; i<=depth; i++) {
    string gap="";
    for (int j=0; j<pow(2,depth-i)-1; j++) {
      gap+=" ";
    }
    string levelNodes = printLevel(root, i, gap);
    cout<<levelNodes<<endl;
  }
}

For example result should be like that:

       4
   1       6
 -   2   5   - 
- - - 3 - - - -

But instead it is:

       4       
   1       6
 -   2   5   -
- - 3 - - -

If I undestand correctly, the recursion stops when the program makes it to the empty leaf and therefore there are lacking " - " on the lower levels in the result. But how do I know how much of those I should draw on the lower levels? How to make this work?

Solution 1

I instrumented the code to see where it went awry (since running a debugger in a browser...), you can see it live here. The reproduced function is:

string printLevel(node *root, int level, string gap) {
  if (root == 0) {
    cout << ".. printLevel - " << root << ": " << gap << "-" << gap << "\n";
    return gap + "-" + gap;
  }
  if (level==1) {
    stringstream out;
    out<<root->value;
    cout << ".. printLevel - " << root << ": " << gap << root->value << gap << "\n";
    return gap + out.str() + gap;
  } else if (level>1) {
    string leftStr = printLevel(root->left, level-1, gap);
    string rightStr = printLevel(root->right, level-1, gap);

    cout << ".. printLevel - " << root << ": '" << leftStr << "', '" << rightStr << "'\n";
    return leftStr + " " + rightStr;
  } else return "";
}

And here is the bit of interesting output:

.. printLevel - <none>: -
.. printLevel - <none>: -
.. printLevel - { 3, <none>, <none> }: 3
.. printLevel - { 2, <none>, { 3, <none>, <none> } }: '-', '3'
.. printLevel - { 1, <none>, { 2, <none>, { 3, <none>, <none> } } }: '-', '- 3'

So, the issue is that you short-circuit whenever root is 0, which is actually an issue: - is not the right output unless level is 1.

The only difference between root being 0 and root not being 0 is that you cannot read the value out of it (and thus get to replace it by -); however you only really read that value when level is 1 (beware, you might try to read left or right too), therefore there is no reason to test for root == 0 unless you are in the level == 1 branch.

Let us slightly reorder things then:

string printLevel(node *root, int level, string gap) {
  if (level==1) {
//    if (root == 0) {
//      cout << ".. printLevel - " << root << ": " << gap << "-" << gap << "\n";
//      return gap + "-" + gap;
//    }
    stringstream out;
    out<<root->value;
    cout << ".. printLevel - " << root << ": " << gap << root->value << gap << "\n";
    return gap + out.str() + gap;
  } else if (level>1) {
//    string leftStr = printLevel(root ? root->left : 0, level-1, gap);
//    string rightStr = printLevel(root ? root->right : 0, level-1, gap);

    cout << ".. printLevel - " << root << ": '" << leftStr << "', '" << rightStr << "'\n";
    return leftStr + " " + rightStr;
  } else return "";
}

Note: I "highlighted" the modified lines with "comments".

And now, the tree prints properly.

Solution 2

void BinaryTree::Display(TreeNode *current, int indent)
{
    if (current != nullptr)
    {
        Display(current->left, indent + 4);
        if (indent > 0)
            cout << setw(indent) << " ";
        cout << current->value << endl;
        Display(current->right, indent + 4);
    }
}

prints tree left to right instead of top down.

            1
        2
    3
        4
5
        6
    7
            8
        12
            18

Solution 3

Here is my code. It prints very well,maybe its not perfectly symmetrical. little description:

• 1st function - prints level by level (root lv -> leaves lv)
• 2nd function - distance from the beginning of new line
• 3rd function - prints nodes and calculates distance between two prints;

---

void Tree::TREEPRINT()
{
    int i = 0;
    while (i <= treeHeight(getroot())){
        printlv(i);
        i++;
        cout << endl;
    }
}

void Tree::printlv(int n){
    Node* temp = getroot();
    int val = pow(2, treeHeight(root) -n+2);
    cout << setw(val) << "";
    prinlv(temp, n, val);
}

void Tree::dispLV(Node*p, int lv, int d)
{
    int disp = 2 * d;
    if (lv == 0){
        if (p == NULL){

            cout << " x ";
            cout << setw(disp -3) << "";
            return;
        }
        else{
            int result = ((p->key <= 1) ? 1 : log10(p->key) + 1);
            cout << " " << p->key << " ";
            cout << setw(disp - result-2) << "";
        }
    }
    else
    {
        if (p == NULL&& lv >= 1){
            dispLV(NULL, lv - 1, d);
            dispLV(NULL, lv - 1, d);
        }
        else{
            dispLV(p->left, lv - 1, d);
            dispLV(p->right, lv - 1, d);
        }
    }
}   

Input:

50-28-19-30-29-17-42-200-160-170-180-240-44-26-27

Output: https://i.stack.imgur.com/TtPXY.png