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Question

want to insert element in binary tree. what is wrong in this code. I have a structure with data, left and right as self referencing structure and root as a global variable of type structure initialized to NULL.

void insert(struct tree_node *root,int data)
{
    if(root == NULL)
    {
        tree_node *new_node = new tree_node();

        new_node->left = NULL;
        new_node->right = NULL;
    }
    else if(root->data > data)
    {
        insert(root->left,data);
    }
    else
    {
    insert(root->right,data);
    }
}

Answer 1

There are several issues with the code you propose here:

• You never set your global variable "root". Also, a global variable is always a bad thing (especially if you plan to use threads).
• You never link the new entries to the existing structure.
• You never set the new entry data

You should probably try this instead:

struct tree_node *insert(struct tree_node *root, int data) 
{
  if(root == NULL) {
    struct tree_node *new_node = new tree_node();

    new_node->data = data;
    new_node->left = NULL;
    new_node->right = NULL;

    return new_node;
  } else if(root->data > data) {
    root->left = insert(root->left, data);
  } else {
    root->right = insert(root->right, data);
  }

  return root;
}

which you can call using

root = insert(root, data);

as shown on this codepad

Answer 2

**you have created new_node but doesn't assign it value .so you can't insert a value to it.

you can also use non recursive function to insert

void insert_nrec(int m)  //for insert in binary tree
{
node *tmp,*par,*ptr;
ptr=root;
par=NULL;
tmp=NULL;
    while(ptr!=NULL)
    {
        par=ptr;
        if(m<ptr->data)
        {   ptr=ptr->lchild;    }
        else if(m>ptr->data)
        {   ptr=ptr->rchild;    }
        else
            {   
                cout<<"duplicate key found::";
                return;             
            }       

    }
tmp=new node;
tmp->data=m;
tmp->lchild=NULL;
tmp->rchild=NULL;
if(par==NULL)
    {
        root=tmp;
    }   
else if(m<par->data)
    {
        par->lchild=tmp;    
    }
else
    {
        par->rchild=tmp;
    }

}

Answer 3

When do you set root to anything other than NULL ?

Once you do, how do you actually change root->left or right ?

Answer 4

I'm just going to rewrite @TheRealNeo's nifty 'functional' answer so that you pass it a reference to the root pointer:

void insert(struct tree_node * &root, int data) 
{
  if(root == NULL) {
    struct tree_node *new_node = new tree_node();

    new_node->data = data;
    new_node->left = NULL;
    new_node->right = NULL;

    root = new_node;
  } else if(root->data > data) {
    insert( root->left, data);
  } else {
    insert( root->right, data);
  }
}

which you call using

insert(root, data);

An advantage of this over the functional one, is there's a good chance the compiler will actually do the recursive calls as 'tail recursions', ie when inserting left, instead of making a recursive call, the code will simply change the 'root' reference to refer to 'root->left' and jump back up to the start of the function.

The explicitly non-recursive form is not as clear, since it involves some pointer-to-pointer stuff, but it's really not so bad:

void insert(struct tree_node * &root, int data) 
{
  struct tree_node **ptrp = &root;

  while(*ptrp != NULL ){
     struct tree_node *np = *ptrp;
     if( np->data > data ){
          ptrp = &np->left;
     }else{
          ptrp = &np->right;
     }
  }
   // new node will be placed at *ptrp, a location
   // currently containing NULL.

  struct tree_node *new_node = new tree_node();

  new_node->data = data;
  new_node->left = NULL;
  new_node->right = NULL;

  *ptrp = new_node;
}