Print a binary tree in a pretty way

Question

Just wondering if I can get some tips on printing a pretty binary tree in the form of:

5      10           11           7                6      3           4           2 

Right now what it prints is:

   2     4     3      6     7     11     10     5 

I know that my example is upside down from what I'm currently printing, which it doesn't matter if I print from the root down as it currently prints. Any tips are very appreciated towards my full question:

How do I modify my prints to make the tree look like a tree?

    //Binary Search Tree Program  #include <iostream> #include <cstdlib> #include <queue> using namespace std;  int i = 0;  class BinarySearchTree {    private:    struct tree_node    {       tree_node* left;       tree_node* right;       int data;    };    tree_node* root;     public:    BinarySearchTree()    {       root = NULL;    }     bool isEmpty() const { return root==NULL; }    void print_inorder();    void inorder(tree_node*);    void print_preorder();    void preorder(tree_node*);    void print_postorder();    void postorder(tree_node*);    void insert(int);    void remove(int); };  // Smaller elements go left // larger elements go right void BinarySearchTree::insert(int d) {    tree_node* t = new tree_node;    tree_node* parent;    t->data = d;    t->left = NULL;    t->right = NULL;    parent = NULL;     // is this a new tree?    if(isEmpty()) root = t;    else    {       //Note: ALL insertions are as leaf nodes       tree_node* curr;       curr = root;       // Find the Node's parent       while(curr)       {          parent = curr;          if(t->data > curr->data) curr = curr->right;          else curr = curr->left;       }        if(t->data < parent->data)       {          parent->left = t;       }       else       {       parent->right = t;       }     } }  void BinarySearchTree::remove(int d) {    //Locate the element    bool found = false;    if(isEmpty())    {       cout<<" This Tree is empty! "<<endl;       return;    }     tree_node* curr;    tree_node* parent;    curr = root;     while(curr != NULL)    {       if(curr->data == d)       {          found = true;          break;       }       else       {          parent = curr;          if(d>curr->data) curr = curr->right;          else curr = curr->left;       }     }     if(!found)     {       cout<<" Data not found! "<<endl;       return;     }       // 3 cases :     // 1. We're removing a leaf node     // 2. We're removing a node with a single child     // 3. we're removing a node with 2 children      // Node with single child     if((curr->left == NULL && curr->right != NULL) || (curr->left != NULL && curr->right == NULL))     {       if(curr->left == NULL && curr->right != NULL)       {          if(parent->left == curr)          {             parent->left = curr->right;             delete curr;          }          else          {             parent->right = curr->left;             delete curr;          }        }        return;     }      //We're looking at a leaf node     if( curr->left == NULL && curr->right == NULL)     {       if(parent->left == curr)       {          parent->left = NULL;       }       else       {          parent->right = NULL;       }       delete curr;       return;     }       //Node with 2 children     // replace node with smallest value in right subtree     if (curr->left != NULL && curr->right != NULL)     {        tree_node* chkr;        chkr = curr->right;        if((chkr->left == NULL) && (chkr->right == NULL))        {          curr = chkr;          delete chkr;          curr->right = NULL;        }        else // right child has children        {          //if the node's right child has a left child          // Move all the way down left to locate smallest element           if((curr->right)->left != NULL)          {             tree_node* lcurr;             tree_node* lcurrp;             lcurrp = curr->right;             lcurr = (curr->right)->left;             while(lcurr->left != NULL)             {                lcurrp = lcurr;                lcurr = lcurr->left;             }             curr->data = lcurr->data;             delete lcurr;             lcurrp->left = NULL;          }          else          {             tree_node* tmp;             tmp = curr->right;             curr->data = tmp->data;             curr->right = tmp->right;             delete tmp;          }        }       return;    }  } void BinarySearchTree::print_postorder() {    postorder(root); }  void BinarySearchTree::postorder(tree_node* p) {    if(p != NULL)    {       if(p->left) postorder(p->left);       if(p->right) postorder(p->right);       cout<<"     "<<p->data<<"\n ";    }    else return; }  int main() {     BinarySearchTree b;     int ch,tmp,tmp1;     while(1)     {        cout<<endl<<endl;        cout<<" Binary Search Tree Operations "<<endl;        cout<<" ----------------------------- "<<endl;        cout<<" 1. Insertion/Creation "<<endl;        cout<<" 2. Printing "<<endl;        cout<<" 3. Removal "<<endl;        cout<<" 4. Exit "<<endl;        cout<<" Enter your choice : ";        cin>>ch;        switch(ch)        {            case 1 : cout<<" Enter Number to be inserted : ";                     cin>>tmp;                     b.insert(tmp);                     i++;                     break;            case 2 : cout<<endl;                     cout<<" Printing "<<endl;                     cout<<" --------------------"<<endl;                     b.print_postorder();                     break;            case 3 : cout<<" Enter data to be deleted : ";                     cin>>tmp1;                     b.remove(tmp1);                     break;            case 4:                     return 0;         }     } } 

Answer 1

In order to pretty-print a tree recursively, you need to pass two arguments to your printing function:

• The tree node to be printed, and
• The indentation level

For example, you can do this:

void BinarySearchTree::postorder(tree_node* p, int indent=0) {     if(p != NULL) {         if(p->left) postorder(p->left, indent+4);         if(p->right) postorder(p->right, indent+4);         if (indent) {             std::cout << std::setw(indent) << ' ';         }         cout<< p->data << "\n ";     } } 

The initial call should be postorder(root);

If you would like to print the tree with the root at the top, move cout to the top of the if.