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Traverse Linked List from middle to left-right order using recursion
  • Difficulty Level : Hard
  • Last Updated : 04 Dec, 2020

Given a Linked List. The task is to traverse the Linked List from middle to left-right order using recursion.
For Example: 
 

If the given Linked List is: 2 -> 5 -> 8 -> 3 -> 7 -> 9 -> 12 -> NULL 
The Middle to left-right order is : 3, 8, 7, 5, 9, 2, 12 
 

Explanation: Middle of the given linked list is ‘3’ so, we start traversing from middle by printing 3 then left and right of 3, so we print 8, 7 then print left of 8 and right of 7, so we print 5, 9 then print left of 5 and right of 9, so we print 2, 12.
Note: If number of node are even in a Linked List then print left right only. For this linked list( contains even number of nodes ) 2 -> 5 -> 8 -> 7 -> 9 -> 12 -> NULL.
The output should be 8, 7, 5, 9, 2, 12.
Examples: 
 

Input: 20 -> 15 -> 23 -> 13 -> 19 -> 32 -> 16 -> 41 -> 11 -> NULL 
Output: 19, 13, 32, 23, 16, 15, 41, 20, 11.
Input: 12 -> 25 -> 51 -> 16 -> 9 -> 90 -> 7 -> 2 -> NULL 
Output: 16, 9, 51, 90, 25, 7, 12, 2. 
 

 



Approach: 
First, calculate the size of the linked list: 
 

  • If size is odd: 
    -> Then go to the (n+1)/2 -th node using recursion.
  • If size is even: 
    -> Then go to the n/2 -th node using recursion. 
     
  • Now print node data and return next node address, do this procedure unless function call stack empty.

Below is the implementation of the above approach: 
 

C++

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// A C++ program to demonstrate
// the printing of Linked List middle
// to left right order
 
#include <bits/stdc++.h>
using namespace std;
 
// A linked list node
class Node {
public:
    int data;
    Node* next;
};
 
// Given a reference (pointer to pointer)
// to the head of a list and an int, appends
// a new node at the end
 
void append(Node** head_ref, int new_data)
{
    // Allocate node
    Node* new_node = new Node();
 
    // Used in step 5
    Node* last = *head_ref;
 
    // Put in the data
    new_node->data = new_data;
 
    // This new node is going to be
    // the last node, so make next of
    // it as NULL
    new_node->next = NULL;
 
    // If the Linked List is empty,
    // then make the new node as head
    if (*head_ref == NULL) {
        *head_ref = new_node;
        return;
    }
 
    // Else traverse till the last node
    while (last->next != NULL)
        last = last->next;
 
    // Change the next of last node
    last->next = new_node;
    return;
}
 
// This function prints contents of
// linked list starting from head
 
void printList(Node* node)
{
    while (node != NULL) {
        cout << " " << node->data;
 
        if (node->next != NULL)
            cout << "->";
        node = node->next;
    }
}
 
// Function to get the size of linked list
int getSize(Node* head)
{
    if (head == NULL)
        return 0;
    return 1 + getSize(head->next);
}
 
// Utility function to print the Linked List
// from middle to left right order
Node* printMiddleToLeftRightUtil(Node* head,
                                 int counter, int lSize)
{
    // Base Condition
    // When size of list is odd
    if (counter == 1 && lSize % 2 != 0) {
 
        // Print node value
        cout << head->data;
 
        // Returns address of next node
        return head->next;
    }
 
    // Base Condition
    // When size of list is even
    else if (counter == 1) {
 
        // Print node value
        // and next node value
        cout << head->data;
        cout << " , " << head->next->data;
 
        // Returns address of next to next node
        return head->next->next;
    }
    else {
 
        // Recursive function call and
        // store return address
        Node* ptr = printMiddleToLeftRightUtil(
            head->next,
            counter - 1,
            lSize);
 
        // Print head data
        cout << " , " << head->data;
 
        // Print ptr data
        cout << " , " << ptr->data;
 
        // Returns address of next node
        return ptr->next;
    }
}
 
// Function to print Middle to
// Left-right order
void printMiddleToLeftRight(Node* head)
{
    // Function call to get the size
    // Of Linked List
    int listSize = getSize(head);
 
    int middle = 0;
 
    // Store middle when Linked
    // List size is odd
    if (listSize % 2 != 0) {
        middle = (listSize + 1) / 2;
    }
 
    // Store middle when Linked
    // List size is even
 
    else {
        middle = listSize / 2;
    }
 
    // Utility function call print
    // Linked List from Middle
    // to left right order
    cout << "Output : ";
 
    printMiddleToLeftRightUtil(head,
                               middle,
                               listSize);
}
 
// Driver code
int main()
{
    // Start with the empty list
    Node* head = NULL;
 
    // Insert 6. So linked list
    // becomes 6->NULL
    append(&head, 6);
 
    // Insert 6. So linked list
    // becomes 6->4->NULL
    append(&head, 4);
    append(&head, 8);
    append(&head, 7);
    append(&head, 9);
    append(&head, 11);
    append(&head, 2);
 
    // After inserting linked list
    // becomes 6->4->8->7->9->11->2->NULL
    cout << "Created Linked list is: ";
 
    // Function to display Linked List content
    printList(head);
    cout << endl;
 
    // Function call print Linked List from
    // Middle to left right order
    printMiddleToLeftRight(head);
    return 0;
}

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Java

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// A Java program to demonstrate
// the printing of Linked List middle
// to left right order
class GFG
{
 
// A linked list node
static class Node
{
    int data;
    Node next;
};
 
// Given a reference (pointer to pointer)
// to the head of a list and an int, appends
// a new node at the end
static Node append(Node head_ref, int new_data)
{
    // Allocate node
    Node new_node = new Node();
 
    // Used in step 5
    Node last = head_ref;
 
    // Put in the data
    new_node.data = new_data;
 
    // This new node is going to be
    // the last node, so make next of
    // it as null
    new_node.next = null;
 
    // If the Linked List is empty,
    // then make the new node as head
    if (head_ref == null)
    {
        head_ref = new_node;
        return head_ref;
    }
 
    // Else traverse till the last node
    while (last.next != null)
        last = last.next;
 
    // Change the next of last node
    last.next = new_node;
    return head_ref;
}
 
// This function prints contents of
// linked list starting from head
static void printList(Node node)
{
    while (node != null)
    {
        System.out.print( " " + node.data);
 
        if (node.next != null)
            System.out.print("->");
        node = node.next;
    }
}
 
// Function to get the size of linked list
static int getSize(Node head)
{
    if (head == null)
        return 0;
    return 1 + getSize(head.next);
}
 
// Utility function to print the Linked List
// from middle to left right order
static Node printMiddleToLeftRightUtil(Node head,
                          int counter, int lSize)
{
    // Base Condition
    // When size of list is odd
    if (counter == 1 && lSize % 2 != 0)
    {
 
        // Print node value
        System.out.print( head.data);
 
        // Returns address of next node
        return head.next;
    }
 
    // Base Condition
    // When size of list is even
    else if (counter == 1)
    {
 
        // Print node value
        // and next node value
        System.out.print(head.data);
        System.out.print( " , " + head.next.data);
 
        // Returns address of next to next node
        return head.next.next;
    }
    else
    {
 
        // Recursive function call and
        // store return address
        Node ptr = printMiddleToLeftRightUtil(head.next,
                                    counter - 1, lSize);
 
        // Print head data
        System.out.print(" , " + head.data);
 
        // Print ptr data
        System.out.print(" , " + ptr.data);
 
        // Returns address of next node
        return ptr.next;
    }
}
 
// Function to print Middle to
// Left-right order
static void printMiddleToLeftRight(Node head)
{
    // Function call to get the size
    // Of Linked List
    int listSize = getSize(head);
 
    int middle = 0;
 
    // Store middle when Linked
    // List size is odd
    if (listSize % 2 != 0)
    {
        middle = (listSize + 1) / 2;
    }
 
    // Store middle when Linked
    // List size is even
    else
    {
        middle = listSize / 2;
    }
 
    // Utility function call print
    // Linked List from Middle
    // to left right order
    System.out.print("Output : ");
 
    printMiddleToLeftRightUtil(head, middle, listSize);
}
 
// Driver code
public static void main(String args[])
{
    // Start with the empty list
    Node head = null;
 
    // Insert 6. So linked list
    // becomes 6.null
    head = append(head, 6);
 
    // Insert 6. So linked list
    // becomes 6.4.null
    head = append(head, 4);
    head = append(head, 8);
    head = append(head, 7);
    head = append(head, 9);
    head = append(head, 11);
    head = append(head, 2);
 
    // After inserting linked list
    // becomes 6.4.8.7.9.11.2.null
    System.out.print("Created Linked list is: ");
 
    // Function to display Linked List content
    printList(head);
    System.out.println();
 
    // Function call print Linked List from
    // Middle to left right order
    printMiddleToLeftRight(head);
}
}
 
// This code is contributed by Arnab Kundu

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Python3

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# A Python3 program to demonstrate
# the printing of Linked List middle
# to left right order
  
# A linked list node
class Node:
     
    def __init__(self):
        self.data = 0
        self.next = None
  
# Given a reference (pointer to pointer)
# to the head of a list and an int, appends
# a new node at the end
def append(head_ref, new_data):
 
    # Allocate node
    new_node = Node();
  
    # Used in step 5
    last = head_ref;
  
    # Put in the data
    new_node.data = new_data;
  
    # This new node is going to be
    # the last node, so make next of
    # it as None
    new_node.next = None;
  
    # If the Linked List is empty,
    # then make the new node as head
    if (head_ref == None):
        head_ref = new_node;
        return head_ref;
  
    # Else traverse till the last node
    while (last.next != None):
        last = last.next;
  
    # Change the next of last node
    last.next = new_node;
     
    return head_ref;
   
# This function prints contents of
# linked list starting from head
def printList(node):
 
    while (node != None):
        print(' ' + str(node.data), end = '')
  
        if (node.next != None):
            print('->', end = '')
             
        node = node.next;
       
# Function to get the size of linked list
def getSize(head):
 
    if (head == None):
        return 0;
     
    return 1 + getSize(head.next);
 
# Utility function to print the Linked List
# from middle to left right order
def printMiddleToLeftRightUtil(head, counter, lSize):
 
    # Base Condition
    # When size of list is odd
    if (counter == 1 and lSize % 2 != 0):
  
        # Print node value
        print(head.data, end = '')
  
        # Returns address of next node
        return head.next;
     
    # Base Condition
    # When size of list is even
    elif (counter == 1):
  
        # Print node value
        # and next node value
        print(str(head.data) + ' , ' + str(head.next.data), end = '')
  
        # Returns address of next to next node
        return head.next.next;
     
    else:
  
        # Recursive function call and
        # store return address
        ptr = printMiddleToLeftRightUtil(head.next, counter - 1,lSize);
  
        # Print head data
        print(' , ' + str(head.data) + ' , ' + str(ptr.data), end = '')
  
        # Returns address of next node
        return ptr.next;
      
# Function to print Middle to
# Left-right order
def printMiddleToLeftRight(head):
 
    # Function call to get the size
    # Of Linked List
    listSize = getSize(head);
  
    middle = 0;
  
    # Store middle when Linked
    # List size is odd
    if (listSize % 2 != 0):
        middle = (listSize + 1) // 2;
  
    # Store middle when Linked
    # List size is even
    else:
        middle = listSize // 2;
      
    # Utility function call print
    # Linked List from Middle
    # to left right order
    print('Output :', end = ' ')
  
    printMiddleToLeftRightUtil(head, middle, listSize);
  
# Driver code
if __name__=='__main__':
     
    # Start with the empty list
    head = None;
  
    # Insert 6. So linked list
    # becomes 6.None
    head = append(head, 6);
  
    # Insert 6. So linked list
    # becomes 6.4.None
    head = append(head, 4);
    head = append(head, 8);
    head = append(head, 7);
    head = append(head, 9)
    head = append(head, 11);
    head = append(head, 2)
     
    # After inserting linked list
    # becomes 6.4.8.7.9.11.2.None
    print("Created Linked list is:", end = ' ')
  
    # Function to display Linked List content
    printList(head);
    print()
  
    # Function call print Linked List from
    # Middle to left right order
    printMiddleToLeftRight(head);
 
    # This code is contributed by rutvik_56

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C#

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// A C# program to demonstrate
// the printing of Linked List middle
// to left right order
using System;
 
public class GFG
{
  
// A linked list node
public class Node
{
    public int data;
    public Node next;
};
  
// Given a reference (pointer to pointer)
// to the head of a list and an int, appends
// a new node at the end
static Node append(Node head_ref, int new_data)
{
    // Allocate node
    Node new_node = new Node();
  
    // Used in step 5
    Node last = head_ref;
  
    // Put in the data
    new_node.data = new_data;
  
    // This new node is going to be
    // the last node, so make next of
    // it as null
    new_node.next = null;
  
    // If the Linked List is empty,
    // then make the new node as head
    if (head_ref == null)
    {
        head_ref = new_node;
        return head_ref;
    }
  
    // Else traverse till the last node
    while (last.next != null)
        last = last.next;
  
    // Change the next of last node
    last.next = new_node;
    return head_ref;
}
  
// This function prints contents of
// linked list starting from head
static void printList(Node node)
{
    while (node != null)
    {
        Console.Write( " " + node.data);
  
        if (node.next != null)
            Console.Write("->");
        node = node.next;
    }
}
  
// Function to get the size of linked list
static int getSize(Node head)
{
    if (head == null)
        return 0;
    return 1 + getSize(head.next);
}
  
// Utility function to print the Linked List
// from middle to left right order
static Node printMiddleToLeftRightUtil(Node head,
                          int counter, int lSize)
{
    // Base Condition
    // When size of list is odd
    if (counter == 1 && lSize % 2 != 0)
    {
  
        // Print node value
        Console.Write( head.data);
  
        // Returns address of next node
        return head.next;
    }
  
    // Base Condition
    // When size of list is even
    else if (counter == 1)
    {
  
        // Print node value
        // and next node value
        Console.Write(head.data);
        Console.Write( " , " + head.next.data);
  
        // Returns address of next to next node
        return head.next.next;
    }
    else
    {
  
        // Recursive function call and
        // store return address
        Node ptr = printMiddleToLeftRightUtil(head.next,
                                    counter - 1, lSize);
  
        // Print head data
        Console.Write(" , " + head.data);
  
        // Print ptr data
        Console.Write(" , " + ptr.data);
  
        // Returns address of next node
        return ptr.next;
    }
}
  
// Function to print Middle to
// Left-right order
static void printMiddleToLeftRight(Node head)
{
    // Function call to get the size
    // Of Linked List
    int listSize = getSize(head);
  
    int middle = 0;
  
    // Store middle when Linked
    // List size is odd
    if (listSize % 2 != 0)
    {
        middle = (listSize + 1) / 2;
    }
  
    // Store middle when Linked
    // List size is even
    else
    {
        middle = listSize / 2;
    }
  
    // Utility function call print
    // Linked List from Middle
    // to left right order
    Console.Write("Output : ");
  
    printMiddleToLeftRightUtil(head, middle, listSize);
}
  
// Driver code
public static void Main(String []args)
{
    // Start with the empty list
    Node head = null;
  
    // Insert 6. So linked list
    // becomes 6.null
    head = append(head, 6);
  
    // Insert 6. So linked list
    // becomes 6.4.null
    head = append(head, 4);
    head = append(head, 8);
    head = append(head, 7);
    head = append(head, 9);
    head = append(head, 11);
    head = append(head, 2);
  
    // After inserting linked list
    // becomes 6.4.8.7.9.11.2.null
    Console.Write("Created Linked list is: ");
  
    // Function to display Linked List content
    printList(head);
    Console.WriteLine();
  
    // Function call print Linked List from
    // Middle to left right order
    printMiddleToLeftRight(head);
}
}
 
// This code is contributed by Rajput-Ji

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Output: 

Created Linked list is:  6-> 4-> 8-> 7-> 9-> 11-> 2
Output : 7 , 8 , 9 , 4 , 11 , 6 , 2

 

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