Connect all nodes to their Left Neighbors in a Binary Tree

Given a Binary Tree, where each node contains an extra empty pointer initially null. The task is to connect all nodes of the binary tree to their left neighbour at the same level using this extra pointer.

Examples:

Input : 
       A
      / \
     B   C
    / \   \
   D   E   F
Output :
       NULL<--A
             / \
     NULL<--B<--C
           / \   \
   NULL<--D<--E<--F


Approach:
We can use Pre-order traversal of tree passing the level of the node at each call. The root node is at level 0. While traversing we store the recently seen node at that level in an array of node pointers. The pre-order traversal ensures that the node in the array at a particular level is left neighbour of upcoming node at the same level.

Below is the implementation of the above approach:

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// CPP program to connect nodes
// at same level using extended
// pre-order traversal
  
#include <bits/stdc++.h>
#include <iostream>
using namespace std;
  
// Binary tree node, with extra pointer leftNeighbour
// to store the neighbour to left nodes
class node {
public:
    int data;
    node* left;
    node* right;
    node* leftNeighbour;
  
    /* Constructor that allocates a new node with the 
       given data and NULL left and right pointers. */
    node(int data)
    {
        this->data = data;
        this->left = NULL;
        this->right = NULL;
        this->leftNeighbour = NULL;
    }
};
  
// Array to store the recent visited 
// node at particulat level represented 
// by indices
node* a[100];
  
// Function to connect nodes using preorder 
// traversal
void connectNodes(node* p, int l)
{
    if (p == NULL)
        return;
  
    // assigning left neighbor
    p->leftNeighbour = a[l];
  
    // updating value of the recent 
    // node at level
    a[l] = p;
    connectNodes(p->left, l + 1);
    connectNodes(p->right, l + 1);
}
  
// Utility function to connect nodes to neighbours
// using preorder traversal
void connectNodesUtil(node* root)
{   
    // Initalize nodes at every level to NULL
    for (int i = 0; i < 100; i++)
        a[i] = NULL;
          
    // Populates next left pointer in all nodes
    connectNodes(root, 0);
      
    // Let us check the values of next left pointers
    cout << "Following are populated leftNeighbour"
            <<" pointers in the tree:\n";
              
    cout << "leftNeighbour of " << root->data << " is "
            << (root->leftNeighbour ? 
                root->leftNeighbour->data : -1) << endl;
                  
    cout << "leftNeighbour of " << root->left->data << " is "
            << (root->left->leftNeighbour ?
                root->left->leftNeighbour->data : -1) << endl;
                  
    cout << "leftNeighbour of " << root->right->data << " is "
            << (root->right->leftNeighbour ? 
                root->right->leftNeighbour->data : -1) << endl;
                  
    cout << "leftNeighbour of " << root->left->left->data << " is "
            << (root->left->left->leftNeighbour ? 
                root->left->left->leftNeighbour->data : -1) << endl;
}
  
// Driver Code
int main()
{
  
    /* Constructed binary tree is 
            10 
            / \ 
           8   2 
          
         
    */
    node* root = new node(10);
    root->left = new node(8);
    root->right = new node(2);
    root->left->left = new node(3);
  
    connectNodesUtil(root);
      
    return 0;
}

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Output:

Following are populated leftNeighbour pointers in the tree:
leftNeighbour of 10 is -1
leftNeighbour of 8 is -1
leftNeighbour of 2 is 8
leftNeighbour of 3 is -1


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