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Connect all nodes to their Left Neighbors in a Binary Tree
  • Last Updated : 25 Jan, 2021

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 neighbor 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 the Pre-order traversal of the 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 neighbor of the upcoming node at the same level.
Below is the implementation of the above approach:  

C++




// 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
          /
         3
    */
    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;
}


Java




// Java program to connect nodes
// at same level using extended
// pre-order traversal
import java.util.*;
 
class GFG
{
 
// Binary tree node, with extra pointer leftNeighbour
// to store the neighbour to left nodes
static class node
{
    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
static node a[] = new node[100];
 
// Function to connect nodes using preorder
// traversal
static 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
static void connectNodesUtil(node root)
{
    // Initalize nodes at every level to null
    for (int i = 0; i < 100; i++)
        a[i] = new node(-1);
         
    // Populates next left pointer in all nodes
    connectNodes(root, 0);
     
    // Let us check the values of next left pointers
    System.out.println( "Following are populated leftNeighbour" +
                        " pointers in the tree:");
             
    System.out.println( "leftNeighbour of " + root.data +
                        " is " + (root.leftNeighbour != null ?
                                  root.leftNeighbour.data : -1));
                 
    System.out.println( "leftNeighbour of " + root.left.data +
                        " is " + (root.left.leftNeighbour != null ?
                                  root.left.leftNeighbour.data : -1));
                 
    System.out.println( "leftNeighbour of " + root.right.data +
                        " is " + (root.right.leftNeighbour != null ?
                                  root.right.leftNeighbour.data : -1) );
                 
    System.out.println( "leftNeighbour of " + root.left.left.data +
                        " is " + (root.left.left.leftNeighbour != null ?
                                  root.left.left.leftNeighbour.data : -1));
}
 
// Driver Code
public static void main(String args[])
{
 
    /* Constructed binary tree is
            10
            / \
        8 2
        /
        3
    */
    node root = new node(10);
    root.left = new node(8);
    root.right = new node(2);
    root.left.left = new node(3);
 
    connectNodesUtil(root);
}
}
 
// This code is contributed by Arnab Kundu


Python3




# Python3 program to connect nodes
# at same level using extended
# pre-order traversal
 
# Binary tree node, with extra
# pointer leftNeighbour to store
# the neighbour to left nodes
class node:
   
    def __init__(self, x):
       
        self.data = x
        self.left = None
        self.right = None
        self.leftNeighbour = None
 
# Array to store the recent visited
# node at particulat level represented
# by indices
a = [None for i in range(100)]
 
# Function to connect nodes using
# preorder traversal
def connectNodes(p, l):
   
    if (p == None):
        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
def connectNodesUtil(root):
 
    # Populates next left
    # pointer in all nodes
    connectNodes(root, 0)
 
    # Let us check the values
    # of next left pointers
    print("Following are populated" +
          "leftNeighbour pointers in" +
          "the tree:")
    x =- 1
 
    if root.leftNeighbour:
        x = root.leftNeighbour.data
 
    print("leftNeighbour of ",
          root.data, " is ", x)
 
    x =- 1
 
    if root.left.leftNeighbour:
        x = root.left.leftNeighbour.data
 
    print("leftNeighbour of ",
          root.left.data, " is ", x)
 
    x =- 1
 
    if root.right.leftNeighbour:
        x = root.right.leftNeighbour.data
 
    print("leftNeighbour of ",
          root.right.data, " is ", x)
 
    x =- 1
 
    if root.left.left.leftNeighbour:
        x = root.left.left.leftNeighbour.data
 
    print("leftNeighbour of ",
          root.left.left.data, " is ", x)
 
# Driver Code
if __name__ == '__main__':
 
    # /* Constructed binary tree is
    #         10
    #         / \
    #        8   2
    #       /
    #      3
    # */
    root = node(10)
    root.left = node(8)
    root.right = node(2)
    root.left.left = node(3)
    connectNodesUtil(root)
     
# This code is contributed by Mohit Kumar 29


C#




// C# program to connect nodes
// at same level using extended
// pre-order traversal
using System;
 
// Binary tree node, with extra pointer leftNeighbour
// to store the neighbour to left nodes
class node
{
    public int data;
    public node left;
    public node right;
    public node leftNeighbour;
  
    /* Constructor that allocates a new node with the
    given data and null left and right pointers. */
    public node(int data)
    {
        this.data = data;
        this.left = null;
        this.right = null;
        this.leftNeighbour = null;
    }
}
class GFG
{
    static node root;
 
  // Array to store the recent visited
    // node at particulat level represented
    // by indices
    static node[] a = new node[100];
   
    // Function to connect nodes using preorder
    // traversal
    static 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
    static void connectNodesUtil(node root)
    {
       
    // Initalize nodes at every level to null
    for (int i = 0; i < 100; i++)
    {
        a[i] = new node(-1);
    }
       
    // Populates next left pointer in all nodes
    connectNodes(root, 0);
     
    // Let us check the values of next left pointers
    Console.WriteLine("Following are populated leftNeighbour" +
                      " pointers in the tree:");
    Console.WriteLine("leftNeighbour of " + root.data +
                      " is " + (root.leftNeighbour != null ?
                                root.leftNeighbour.data : -1));
    Console.WriteLine("leftNeighbour of " + root.left.data +
                      " is " + (root.left.leftNeighbour != null ?
                                root.left.leftNeighbour.data : -1));
    Console.WriteLine("leftNeighbour of " + root.right.data +
                      " is " + (root.right.leftNeighbour != null ?
                                root.right.leftNeighbour.data : -1) );
    Console.WriteLine( "leftNeighbour of " + root.left.left.data +
                      " is " + (root.left.left.leftNeighbour != null ?
                                root.left.left.leftNeighbour.data : -1));
    }
   
    // Driver Code
    static public void Main ()
    {
       
        /* Constructed binary tree is
            10
            / \
        8 2
        /
        3
    */
        GFG.root = new node(10);
        GFG.root.left = new node(8);
        GFG.root.right = new node(2);
        GFG.root.left.left = new node(3);
        connectNodesUtil(root);
    }
}
 
// This code is contributed by avanitrachhadiya2155


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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