C++ Program For Rearranging A Linked List In Zig-Zag Fashion

Last Updated : 22 Aug, 2024

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

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.

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)  
    { 
        // "<" relation expected  
        if (flag)  
        { 
            /* 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); 
        } 
        // ">" relation expected  
        else 
        { 
            /* 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; 
  
        // flip flag for reverse checking  
        flag = !flag;  
    } 
} 
  
// 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 off 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 code 
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 "); 
    printList(head); 
  
    zigZagList(head); 
  
    printf("Zig Zag Linked list "); 
    printList(head); 
  
    return (0); 
} 

Output:

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

Time Complexity: O(N), as we are using a loop for traversing the linked list.

Auxiliary Space: O(1), as we are not using extra space.

Please refer complete article on Rearrange a Linked List in Zig-Zag fashion for more details!

C++ Program For Rotating A Linked List

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