How to store an array of LinkedList heads in C++? - data-structures

Question

Given a binary tree, I'm trying to make a linked list of elements for each depth. That makes D lists for depth D. I'm doing a non-recursive implementation and using level order traversal in my C++ code. Though I don't see any error while compilation, I know that my heads are not storing in the array of pointers. Please have a look at my code. Any help or suggestion would be great since I'm new to data structures. Thanks!
#include <iostream>
#include <queue>
using namespace std;
struct BTnode
{
int data;
BTnode* left;
BTnode* right;
};
struct LLnode
{
int data;
LLnode* next;
};
BTnode* newNode(int data)
{
BTnode* node = new BTnode;
node->data = data;
node->left = NULL;
node->right = NULL;
return node;
}
void MakeLL(LLnode* &head, int data)
{
LLnode* temp = new LLnode;
temp->data = data;
if (head == NULL)
{
head = temp;
temp->next = NULL;
}
else
{
temp->next = head;
head = temp;
}
}
LLnode** LevelElementsLinkedlist(BTnode* &root)
{
if (root == NULL)
return NULL;
queue<BTnode*>nodeQ;
nodeQ.push(root);
int level = 0;
LLnode **arr;
while(1)
{
int count = nodeQ.size();
if (count == 0)
break;
LLnode* head = NULL;
arr[level] = head;
level ++;
while (count > 0)
{
BTnode* node = nodeQ.front();
MakeLL(head,node->data);
nodeQ.pop();
if (node->left)
nodeQ.push(node->left);
if (node->right)
nodeQ.push(node->right);
count--;
}
}
return arr;
}
void printLL(LLnode* head)
{
if (head == NULL)
return;
LLnode* temp = head;
while (temp != NULL)
{
cout << temp->data << " ";
temp = temp->next;
}
}
int main()
{
BTnode* root = newNode(1);
root->left = newNode(2);
root->right = newNode(3);
root->left->left = newNode(40);
root->left->right = newNode(5);
root->right->left = newNode(60);
root->right->right = newNode(7);
root->left->left->left = newNode(8);
root->left->left->right = newNode(9);
root->left->right->left = newNode(10);
root->left->right->right = newNode(11);
root->right->left->left = newNode(12);
root->right->left->right = newNode(13);
root->right->right->left = newNode(14);
root->right->right->right = newNode(15);
LLnode** arr = LevelElementsLinkedlist(root);
int n = sizeof(arr)/sizeof(arr[0]);
cout << n;
for (int i = 0; i < n; i++)
{
printLL(arr[i]);
cout << endl;
}
cout << endl;
return 0;
}

Answer 1

There are a few problem that I noticed in your code.
In the function LevelElementsLinkedlist, you are not allocating space for arr. It should be LLnode *arr = new LLnode *[100].
Check the line where you have written the following:
LLnode* head = NULL;
arr[level] = head;
You are assigning NULL to head and then adding it to the array. Your whole array will contain nothing else but NULLs.
Try fixing these issues and try executing your code again.