Rearrange a Linked List in Zig-Zag fashion

Given a linked list, rearrange it such that the converted list should be of the form a < b > c < d > e < f … where a, b, c… are consecutive data nodes of the linked list.

Examples:

Input:  1->2->3->4
Output: 1->3->2->4 
Explanation : 1 and 3 should come first before 2 and 4 in zig-zag fashion, So resultant linked-list will be 1->3->2->4. 

Input:  11->15->20->5->10
Output: 11->20->5->15->10 

We strongly recommend that you click here and practice it, before moving on to the solution.

A simple approach to do this is to sort the linked list using merge sort and then swap alternate, but that requires O(n Log n) time complexity. Here n is a number of elements in the linked list.

An efficient approach that requires O(n) time is, using a single scan similar to bubble sort and then maintain a flag for representing which order () currently we are. If the current two elements are not in that order then swap those elements otherwise not. Please refer to this for a detailed explanation of the swapping order. 

Implementation:

C++

// C++ program to arrange linked list in zigzag fashion
#include <bits/stdc++.h>
using namespace std;

/* Link list Node */
struct Node {
    int data;
    struct Node* next;
};

// This function distributes the Node in zigzag fashion
void zigZagList(Node* head)
{
    // If flag is true, then next node should be greater in
    // the desired output.
    bool flag = true;

    // Traverse linked list starting from head.
    Node* current = head;
    while (current->next != NULL) {
        if (flag) /* "<" relation expected */
        {
            // If we have a situation like A > B > C where
            // A, B and C are consecutive Nodes in list we
            // get A > B < C by swapping B and C
            if (current->data > current->next->data)
                swap(current->data, current->next->data);
        }
        else /* ">" relation expected */
        {
            // If we have a situation like A < B < C where
            // A, B and C  are consecutive Nodes in list we
            // get A < C > B by swapping B and C
            if (current->data < current->next->data)
                swap(current->data, current->next->data);
        }

        current = current->next;
        flag = !flag; /* flip flag for reverse checking */
    }
}

/* UTILITY FUNCTIONS */
/* Function to push a Node */
void push(Node** head_ref, int new_data)
{
    /* allocate Node */
    struct Node* new_Node = new Node;

    /* put in the data  */
    new_Node->data = new_data;

    /* link the old list of the new Node */
    new_Node->next = (*head_ref);

    /* move the head to point to the new Node */
    (*head_ref) = new_Node;
}

/* Function to print linked list */
void printList(struct Node* Node)
{
    while (Node != NULL) {
        printf("%d->", Node->data);
        Node = Node->next;
    }
    printf("NULL");
}

/* Driver program to test above function*/
int main(void)
{
    /* Start with the empty list */
    struct Node* head = NULL;

    // create a list 4 -> 3 -> 7 -> 8 -> 6 -> 2 -> 1
    // answer should be -> 3  7  4  8  2  6  1
    push(&head, 1);
    push(&head, 2);
    push(&head, 6);
    push(&head, 8);
    push(&head, 7);
    push(&head, 3);
    push(&head, 4);

    printf("Given linked list \n");
    printList(head);

    zigZagList(head);

    printf("\nZig Zag Linked list \n");
    printList(head);

    return (0);
}

// This code is contributed by Sania Kumari Gupta (kriSania804)

C

// C program to arrange linked list in zigzag fashion
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>

/* Link list Node */
typedef struct Node {
    int data;
    struct Node* next;
} Node;

// This function swaps values pointed by xp and yp
void swap(int* xp, int* yp)
{
    int temp = *xp;
    *xp = *yp;
    *yp = temp;
}

// This function distributes the Node in zigzag fashion
void zigZagList(Node* head)
{
    // If flag is true, then next node should be greater in
    // the desired output.
    bool flag = true;

    // Traverse linked list starting from head.
    Node* current = head;
    while (current->next != NULL) {
        if (flag) /* "<" relation expected */
        {
            // If we have a situation like A > B > C where
            // A, B and C are consecutive Nodes in list we
            // get A > B < C by swapping B and C
            if (current->data > current->next->data)
                swap(¤t->data, ¤t->next->data);
        }
        else /* ">" relation expected */
        {
            // If we have a situation like A < B < C where
            // A, B and C  are consecutive Nodes in list we
            // get A < C > B by swapping B and C
            if (current->data < current->next->data)
                swap(¤t->data, ¤t->next->data);
        }

        current = current->next;
        flag = !flag; /* flip flag for reverse checking */
    }
}

/* UTILITY FUNCTIONS */
/* Function to push a Node */
void push(Node** head_ref, int new_data)
{
    /* allocate Node */
    struct Node* new_Node = (Node*)malloc(sizeof(Node));

    /* put in the data  */
    new_Node->data = new_data;

    /* link the old list of the new Node */
    new_Node->next = (*head_ref);

    /* move the head to point to the new Node */
    (*head_ref) = new_Node;
}

/* Function to print linked list */
void printList(struct Node* Node)
{
    while (Node != NULL) {
        printf("%d->", Node->data);
        Node = Node->next;
    }
    printf("NULL");
}

/* Driver program to test above function*/
int main(void)
{
    /* Start with the empty list */
    struct Node* head = NULL;

    // create a list 4 -> 3 -> 7 -> 8 -> 6 -> 2 -> 1
    // answer should be -> 3  7  4  8  2  6  1
    push(&head, 1);
    push(&head, 2);
    push(&head, 6);
    push(&head, 8);
    push(&head, 7);
    push(&head, 3);
    push(&head, 4);

    printf("Given linked list \n");
    printList(head);

    zigZagList(head);

    printf("\nZig Zag Linked list \n");
    printList(head);

    return (0);
}

// This code is contributed by Sania Kumari Gupta (kriSania804)

Java

// Java program to arrange
// linked list in zigzag fashion
class GfG {

    /* Link list Node */
    static class Node {
        int data;
        Node next;
    }
    static Node head = null;
    static int temp = 0;

    // This function distributes
    // the Node in zigzag fashion
    static void zigZagList(Node head)
    {
        // If flag is true, then
        // next node should be greater
        // in the desired output.
        boolean flag = true;

        // Traverse linked list starting from head.
        Node current = head;
        while (current != null && current.next != null) {
            if (flag == true) /* "<" relation expected */
            {
                /* If we have a situation like A > B > C
            where A, B and C are consecutive Nodes
            in list we get A > B < C by swapping B
            and C */
                if (current.data > current.next.data) {
                    temp = current.data;
                    current.data = current.next.data;
                    current.next.data = temp;
                }
            }
            else /* ">" relation expected */
            {
                /* If we have a situation like A < B < C where
            A, B and C are consecutive Nodes in list we
            get A < C > B by swapping B and C */
                if (current.data < current.next.data) {
                    temp = current.data;
                    current.data = current.next.data;
                    current.next.data = temp;
                }
            }

            current = current.next;

            /* flip flag for reverse checking */
            flag = !(flag);
        }
    }

    /* UTILITY FUNCTIONS */
    /* Function to push a Node */
    static void push(int new_data)
    {
        /* allocate Node */
        Node new_Node = new Node();

        /* put in the data */
        new_Node.data = new_data;

        /* link the old list of the new Node */
        new_Node.next = (head);

        /* move the head to point to the new Node */
        (head) = new_Node;
    }

    /* Function to print linked list */
    static void printList(Node Node)
    {
        while (Node != null) {
            System.out.print(Node.data + "->");
            Node = Node.next;
        }
        System.out.println("NULL");
    }

    /* Driver code*/
    public static void main(String[] args)
    {
        /* Start with the empty list */
        // Node head = null;

        // create a list 4 -> 3 -> 7 -> 8 -> 6 -> 2 -> 1
        // answer should be -> 3 7 4 8 2 6 1
        push(1);
        push(2);
        push(6);
        push(8);
        push(7);
        push(3);
        push(4);

        System.out.println("Given linked list ");
        printList(head);

        zigZagList(head);

        System.out.println("Zig Zag Linked list ");
        printList(head);
    }
}

// This code is contributed by
// Prerna Saini.

Python

# Python code to rearrange linked list in zig zag fashion

# Node class
class Node:

    # Constructor to initialize the node object
    def __init__(self, data):
        self.data = data
        self.next = None


# This function distributes the Node in zigzag fashion
def zigZagList(head):

    # If flag is true, then next node should be greater
    # in the desired output.
    flag = True

    # Traverse linked list starting from head.
    current = head
    while (current.next != None):

        if (flag): # "<" relation expected

            # If we have a situation like A > B > C
            # where A, B and C are consecutive Nodes
            # in list we get A > B < C by swapping B
            # and C
            if (current.data > current.next.data):
                t = current.data
                current.data = current.next.data
                current.next.data = t


        else :# ">" relation expected

            # If we have a situation like A < B < C where
            # A, B and C are consecutive Nodes in list we
            # get A < C > B by swapping B and C
            if (current.data < current.next.data):
                t = current.data
                current.data = current.next.data
                current.next.data = t

        current = current.next
        if(flag):
            flag = False # flip flag for reverse checking
        else:
            flag = True
    return head

# function to insert a Node in
# the linked list at the beginning.
def push(head, k):

    tem = Node(0)
    tem.data = k
    tem.next = head
    head = tem
    return head

# function to display Node of linked list.
def display( head):

    curr = head
    while (curr != None):
        print( curr.data, "->", end =" ")
        curr = curr.next

    print("None")

# Driver code

head = None

# create a list 4 -> 3 -> 7 -> 8 -> 6 -> 2 -> 1
# answer should be -> 3 7 4 8 2 6 1
head = push(head, 1)
head = push(head, 2)
head = push(head, 6)
head = push(head, 8)
head = push(head, 7)
head = push(head, 3)
head = push(head, 4)

print("Given linked list \n")
display(head)

head = zigZagList(head)

print("\nZig Zag Linked list \n")
display(head)

# This code is contributed by Arnab Kundu

C#

// C# program to arrange
// linked list in zigzag fashion
using System;

class GfG {

    /* Link list Node */
    class Node {
        public int data;
        public Node next;
    }
    static Node head = null;
    static int temp = 0;

    // This function distributes
    // the Node in zigzag fashion
    static void zigZagList(Node head)
    {
        // If flag is true, then
        // next node should be greater
        // in the desired output.
        bool flag = true;

        // Traverse linked list starting from head.
        Node current = head;
        while (current != null && current.next != null) {
            if (flag == true) /* "<" relation expected */
            {
                /* If we have a situation like A > B > C
                where A, B and C are consecutive Nodes
                in list we get A > B < C by swapping B
                and C */
                if (current != null && current.next != null && current.data > current.next.data) {
                    temp = current.data;
                    current.data = current.next.data;
                    current.next.data = temp;
                }
            }
            else /* ">" relation expected */
            {
                /* If we have a situation like A < B < C where
                A, B and C are consecutive Nodes in list we
                get A < C > B by swapping B and C */
                if (current != null && current.next != null && current.data < current.next.data) {
                    temp = current.data;
                    current.data = current.next.data;
                    current.next.data = temp;
                }
            }

            current = current.next;

            /* flip flag for reverse checking */
            flag = !(flag);
        }
    }

    /* UTILITY FUNCTIONS */
    /* Function to push a Node */
    static void push(int new_data)
    {
        /* allocate Node */
        Node new_Node = new Node();

        /* put in the data */
        new_Node.data = new_data;

        /* link the old list of the new Node */
        new_Node.next = (head);

        /* move the head to point to the new Node */
        (head) = new_Node;
    }

    /* Function to print linked list */
    static void printList(Node Node)
    {
        while (Node != null) {
            Console.Write(Node.data + "->");
            Node = Node.next;
        }
        Console.WriteLine("NULL");
    }

    /* Driver code*/
    public static void Main()
    {
        /* Start with the empty list */
        // Node head = null;

        // create a list 4 -> 3 -> 7 -> 8 -> 6 -> 2 -> 1
        // answer should be -> 3 7 4 8 2 6 1
        push(1);
        push(2);
        push(6);
        push(8);
        push(7);
        push(3);
        push(4);

        Console.WriteLine("Given linked list ");
        printList(head);

        zigZagList(head);

        Console.WriteLine("Zig Zag Linked list ");
        printList(head);
    }
}
/* This code is contributed PrinciRaj1992 */

Javascript

<script>

// Javascript program to arrange
// linked list in zigzag fashion

    /* Link list Node */
     class Node {
            constructor() {
                this.data = 0;
                this.next = null;
            }
        }

    var head = null;
    var temp = 0;

    // This function distributes
    // the Node in zigzag fashion
    function zigZagList(head) {
        // If flag is true, then
        // next node should be greater
        // in the desired output.
        var flag = true;

        // Traverse linked list starting from head.
        var current = head;
        while (current != null && current.next != null) {
            if (flag == true) /* "<" relation expected */
            {
                /*
                 * If we have a situation like A > B > C
                 where A, B and C are consecutive Nodes
                 * in list we get A > B < C by swapping B and C
                 */
                if (current.data > current.next.data) {
                    temp = current.data;
                    current.data = current.next.data;
                    current.next.data = temp;
                }
            } else /* ">" relation expected */
            {
                /*
                 * If we have a situation like A < B < C
                 where A, B and C are consecutive Nodes
                 * in list we get A < C > B by swapping B and C
                 */
                if (current.data < current.next.data) {
                    temp = current.data;
                    current.data = current.next.data;
                    current.next.data = temp;
                }
            }

            current = current.next;

            /* flip flag for reverse checking */
            flag = !(flag);
        }
    }

    /* UTILITY FUNCTIONS */
    /* Function to push a Node */
    function push(new_data) {
        /* allocate Node */
        var new_Node = new Node();

        /* put in the data */
        new_Node.data = new_data;

        /* link the old list of the new Node */
        new_Node.next = (head);

        /* move the head to point to the new Node */
        (head) = new_Node;
    }

    /* Function to print linked list */
    function printList(node) {
        while (node != null) {
            document.write(node.data + "->");
    node = node.next;
        }
        document.write("NULL<br/>");
    }

    /* Driver code */

        /* Start with the empty list */
        // Node head = null;

        // create a list 4 -> 3 -> 7 -> 8 -> 6 -> 2 -> 1
        // answer should be -> 3 7 4 8 2 6 1
        push(1);
        push(2);
        push(6);
        push(8);
        push(7);
        push(3);
        push(4);

        document.write("Given linked list <br/>");
        printList(head);

        zigZagList(head);

        document.write("Zig Zag Linked list <br/>");
        printList(head);

// This code contributed by gauravrajput1

</script>

Output

Given linked list 
4->3->7->8->6->2->1->NULL
Zig Zag Linked list 
3->7->4->8->2->6->1->NULL

Another Approach:
In the above code, the push function pushes the node at the front of the linked list, the code can be easily modified for pushing the node at the end of the list. Another thing to note is, swapping of data between two nodes is done by swap by value not swap by links for simplicity, for the swap by links technique please see this.

This can be also be done recursively. The idea remains the same, let us suppose the value of the flag determines the condition we need to check for comparing the current element. So, if the flag is 0 (or false) the current element should be smaller than the next and if the flag is 1 ( or true ) then the current element should be greater than the next. If not, swap the values of nodes.

Implementation:

C++

// C++ program to arrange linked list
// in zigzag fashion
#include <iostream>
using namespace std;

// a linked list node
struct node {
    int data;
    node* next;
};

/* Function to push a Node */
void push(node** head_ref, int new_data)
{
    node* new_Node = (node*)malloc(sizeof(node));
    new_Node->data = new_data;
    new_Node->next = (*head_ref);
    (*head_ref) = new_Node;
}

// Rearrange the linked list in zig zag way
node* zigzag(node* head, bool flag)
{
    if (!head || !head->next)
        return head;
    if (flag == 1) {
        if (head->data > head->next->data)
            swap(head->data, head->next->data);
        return zigzag(head->next, !flag);
    }
    else {
        if (head->data < head->next->data)
            swap(head->data, head->next->data);
        return zigzag(head->next, !flag);
    }
}

// fun to print list
void printList(node* head)
{
    while (head) {
        cout << head->data << "-> ";
        head = head->next;
    }
    cout << "NULL";
}

// main fun
int main()
{

    node* head = NULL;
    push(&head, 10);
    push(&head, 5);
    push(&head, 20);
    push(&head, 15);
    push(&head, 11);

    printList(head);
    cout << endl;

    zigzag(head, 1);

    cout << "LL in zig zag fashion : " << endl;
    printList(head);
    return 0;
}

// This code is contributed by Upendra

C

// C program to arrange linked list in zigzag fashion
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>

// a linked list node
typedef struct node {
    int data;
    struct node* next;
} node;

void push(node** head_ref, int new_data)
{
    node* new_Node = (node*)malloc(sizeof(node));
    new_Node->data = new_data;
    new_Node->next = (*head_ref);
    (*head_ref) = new_Node;
}

// function to swap the elements
void swap(int* xp, int* yp)
{
    int temp = *xp;
    *xp = *yp;
    *yp = temp;
}

// Rearrange the linked list in zig zag way
node* zigzag(node* head, bool flag)
{
    if (!head || !head->next)
        return head;
    if (flag == 1) {
        if (head->data > head->next->data)
            swap(&head->data, &head->next->data);
        return zigzag(head->next, !flag);
    }
    else {
        if (head->data < head->next->data)
            swap(&head->data, &head->next->data);
        return zigzag(head->next, !flag);
    }
}

// fun to print list
void printList(node* head)
{
    while (head) {
        printf("%d-> ", head->data);
        head = head->next;
    }
    printf("NULL");
}

// main fun
int main()
{

    node* head = NULL;
    push(&head, 10);
    push(&head, 5);
    push(&head, 20);
    push(&head, 15);
    push(&head, 11);

    printList(head);
    printf("\n");

    zigzag(head, 1);

    printf("LL in zig zag fashion : \n");
    printList(head);
    return 0;
}

// This code is contributed by Aditya Kumar (adityakumar129)

Java

// Java program for the above approach
import java.io.*;

// Node class
class Node {
    int data;
    Node next;
    Node(int data) { this.data = data; }
}

public class GFG {

    private Node head;

    // Print Linked List
    public void printLL()
    {
        Node t = head;
        while (t != null) {
            System.out.print(t.data + " ->");
            t = t.next;
        }
        System.out.println();
    }

    // Swap both nodes
    public void swap(Node a, Node b)
    {
        if (a == null || b == null)
            return;
        int temp = a.data;
        a.data = b.data;
        b.data = temp;
    }

    // Rearrange the linked list
    // in zig zag way
    public Node zigZag(Node node, int flag)
    {
        if (node == null || node.next == null) {
            return node;
        }
        if (flag == 0) {
            if (node.data > node.next.data) {
                swap(node, node.next);
            }
            return zigZag(node.next, 1);
        }
        else {
            if (node.data < node.next.data) {
                swap(node, node.next);
            }
            return zigZag(node.next, 0);
        }
    }

    // Driver Code
    public static void main(String[] args)
    {
        GFG lobj = new GFG();
        lobj.head = new Node(11);
        lobj.head.next = new Node(15);
        lobj.head.next.next = new Node(20);
        lobj.head.next.next.next = new Node(5);
        lobj.head.next.next.next.next = new Node(10);
        lobj.printLL();

        // 0 means the current element
        // should be smaller than the next
        int flag = 0;
        lobj.zigZag(lobj.head, flag);
        System.out.println("LL in zig zag fashion : ");
        lobj.printLL();
    }
}

Python3

# Python program for the above approach// Node class
class Node:

    # Constructor to initialize the node object
    def __init__(self, data):
        self.data = data
        self.next = None


head = None

# Print Linked List


def printLL():
    t = head
    while (t != None):
        print(t.data, end=" ->")
        t = t.next
    print()

# Swap both nodes


def swap(a, b):
    if(a == None or b == None):
        return
    temp = a.data
    a.data = b.data
    b.data = temp

# Rearrange the linked list
# in zig zag way


def zigZag(node, flag):
    if(node == None or node.next == None):
        return node
    if (flag == 0):
        if (node.data > node.next.data):
            swap(node, node.next)
        return zigZag(node.next, 1)

    else:
        if (node.data < node.next.data):
            swap(node, node.next)
        return zigZag(node.next, 0)


# Driver Code
head = Node(11)
head.next = Node(15)
head.next.next = Node(20)
head.next.next.next = Node(5)
head.next.next.next.next = Node(10)
printLL()

# 0 means the current element
# should be smaller than the next
flag = 0
zigZag(head, flag)
print("LL in zig zag fashion : ")
printLL()

# This code is contributed by avanitrachhadiya2155.

C#

// C# program for the above approach
using System;

// Node class
public class Node {
    public int data;
    public Node next;
    public

        Node(int data)
    {
        this.data = data;
    }
}

public class GFG {

    private Node head;

    // Print Linked List
    public void printLL()
    {
        Node t = head;
        while (t != null) {
            Console.Write(t.data + " ->");
            t = t.next;
        }
        Console.WriteLine();
    }

    // Swap both nodes
    public void swap(Node a, Node b)
    {
        if (a == null || b == null)
            return;
        int temp = a.data;
        a.data = b.data;
        b.data = temp;
    }

    // Rearrange the linked list
    // in zig zag way
    public Node zigZag(Node node, int flag)
    {
        if (node == null || node.next == null) {
            return node;
        }
        if (flag == 0) {
            if (node.data > node.next.data) {
                swap(node, node.next);
            }
            return zigZag(node.next, 1);
        }
        else {
            if (node.data < node.next.data) {
                swap(node, node.next);
            }
            return zigZag(node.next, 0);
        }
    }

    // Driver Code
    public static void Main(String[] args)
    {
        GFG lobj = new GFG();
        lobj.head = new Node(11);
        lobj.head.next = new Node(15);
        lobj.head.next.next = new Node(20);
        lobj.head.next.next.next = new Node(5);
        lobj.head.next.next.next.next = new Node(10);
        lobj.printLL();

        // 0 means the current element
        // should be smaller than the next
        int flag = 0;
        lobj.zigZag(lobj.head, flag);
        Console.WriteLine("LL in zig zag fashion : ");
        lobj.printLL();
    }
}

// This code is contributed by umadevi9616

Javascript

<script>
// javascript program for the above approach// Node class
class Node {
    constructor(val) {
        this.data = val;
        this.next = null;
    }
}

var head;

    // Print Linked List
     function printLL() {
var t = head;
        while (t != null) {
            document.write(t.data + " ->");
            t = t.next;
        }
        document.write();
    }

    // Swap both nodes
     function swap(a,  b) {
        if (a == null || b == null)
            return;
        var temp = a.data;
        a.data = b.data;
        b.data = temp;
    }

    // Rearrange the linked list
    // in zig zag way
     function zigZag(node , flag) {
        if (node == null || node.next == null) {
            return node;
        }
        if (flag == 0) {
            if (node.data > node.next.data) {
                swap(node, node.next);
            }
            return zigZag(node.next, 1);
        } else {
            if (node.data < node.next.data) {
                swap(node, node.next);
            }
            return zigZag(node.next, 0);
        }
    }

    // Driver Code

        head = new Node(11);
        head.next = new Node(15);
        head.next.next = new Node(20);
        head.next.next.next = new Node(5);
        head.next.next.next.next = new Node(10);
        printLL();

        // 0 means the current element
        // should be smaller than the next
        var flag = 0;
        zigZag(head, flag);
        document.write("<br/>LL in zig zag fashion : <br/>");
        printLL();

// This code contributed by umadevi9616
</script>

Output

11 ->15 ->20 ->5 ->10 ->
LL in zig zag fashion : 
11 ->20 ->5 ->15 ->10 ->

Complexity Analysis:

• Time Complexity: O(n). 
  Traversal of the list is done only once, and it has ‘n’ elements.
• Auxiliary Space: O(n). 
  O(n) extra space data structure for storing values.