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Question

Please see BST codes below. It only outputs "5". what did I do wrong?

#include <iostream>

class bst {
 public:
  bst(const int& numb) : root(new node(numb)) {}

  void insert(const int& numb) {
    root->insert(new node(numb), root);
  }

  void inorder() {
    root->inorder(root);
  }

 private:
  class node {
   public:
    node(const int& numb) : left(NULL), right(NULL) {
      value = numb;
    }

    void insert(node* insertion, node* position) {
      if (position == NULL) position = insertion;
      else if (insertion->value > position->value)
        insert(insertion, position->right);
      else if (insertion->value < position->value)
        insert(insertion, position->left);
    }

    void inorder(node* tree) {
      if (tree == NULL)
        return;
      inorder(tree->left);
      std::cout << tree->value << std::endl;
      inorder(tree->right); 
    }
  private:
    node* left;
    node* right;
    int value;
  };

  node* root;
};

int main() {
  bst tree(5);
  tree.insert(4);
  tree.insert(2);
  tree.insert(10);
  tree.insert(14);
  tree.inorder();
  return 0;
}

Answer 1

use reference:

void insert(node* insertion, node* &position)

void inorder(node* &tree) {

Answer 2

It's because you never set the values of the left and right fields of root .

Somewhere you have to say, for a given node, n :

n->left = ...
n->right = ...

You never did this. So you ended up with a single node tree. Your root has two null children.

You can get sneaky about this, too: if you do what @user1431015 suggests, and pass the child pointers by reference, then the assignment to the reference paraemter ( position ) will do the trick. Passing them by value, as you did, only assigns to a local variable, and not to the tree itself.

Answer 3

Your insert never ends up doing anything in most cases. The base case of your recurstion is:

void insert(node* insertion, node* position) {
     if (position == NULL) position = insertion;

But all 'position' is is a locally scoped pointer value. Assigning to it will have no effect once your function exits.

What you need to do is make the position parameter a reference to pointer. IN other words, make it of type node*& . Then the assignment will stick after you exit the function.