Unrolled Linked List | Set 1 (Introduction)

05 Jul, 2022
Like array and linked list, the unrolled Linked List is also a linear data structure and is a variant of a linked list. 
Why do we need unrolled linked list?
One of the biggest advantages of linked lists over arrays is that inserting an element at any location takes only O(1). However, the catch here is that to search an element in a linked list takes O(n). So to solve the problem of searching i.e reducing the time for searching the element the concept of unrolled linked lists was put forward. The unrolled linked list covers the advantages of both array and linked list as it reduces the memory overhead in comparison to simple linked lists by storing multiple elements at each node and it also has the advantage of fast insertion and deletion as that of a linked list.
Advantages:
Because of the Cache behavior, linear search is much faster in unrolled linked lists.
In comparison to the ordinary linked list, it requires less storage space for pointers/references.
It performs operations like insertion, deletion, and traversal more quickly than ordinary linked lists (because search is faster).
Disadvantages:
The overhead per node is comparatively high than singly-linked lists. Refer to an example node in the below code
Example: Let say we are having 8 elements so sqrt(8)=2.82 which rounds off to 3. So each block will store 3 elements. Hence, to store 8 elements 3 blocks will be created out of which the first two blocks will store 3 elements and the last block would store 2 elements.
How searching becomes better in unrolled linked lists?
So taking the above example if we want to search for the 7th element in the list we traverse the list of blocks to the one that contains the 7th element. It takes only O(sqrt(n)) since we found it through not going more than sqrt(n) blocks. 
Simple Implementation:
The below program creates a simple unrolled linked list with 3 nodes containing a variable number of elements in each. It also traverses the created list.
C++
C
Java
Python3
C#
Javascript
// C++ program to implement unrolled linked list// and traversing it.#include <bits/stdc++.h>using namespace std;#define maxElements 4 // Unrolled Linked List Nodeclass Node{    public:    int numElements;    int array[maxElements];    Node *next;}; /* Function to traverse an unrolled linked listand print all the elements*/void printUnrolledList(Node *n){    while (n != NULL)    {        // Print elements in current node        for (int i=0; i<n->numElements; i++)            cout<<n->array[i]<<" ";         // Move to next node        n = n->next;    }} // Program to create an unrolled linked list// with 3 Nodesint main(){    Node* head = NULL;    Node* second = NULL;    Node* third = NULL;     // allocate 3 Nodes    head = new Node();    second = new Node();    third = new Node();     // Let us put some values in second node (Number    // of values must be less than or equal to    // maxElement)    head->numElements = 3;    head->array[0] = 1;    head->array[1] = 2;    head->array[2] = 3;     // Link first Node with the second Node    head->next = second;     // Let us put some values in second node (Number    // of values must be less than or equal to    // maxElement)    second->numElements = 3;    second->array[0] = 4;    second->array[1] = 5;    second->array[2] = 6;     // Link second Node with the third Node    second->next = third;     // Let us put some values in third node (Number    // of values must be less than or equal to    // maxElement)    third->numElements = 3;    third->array[0] = 7;    third->array[1] = 8;    third->array[2] = 9;    third->next = NULL;     printUnrolledList(head);     return 0;} // This is code is contributed by rathbhupendra
// C program to implement unrolled linked list// and traversing it.#include<stdio.h>#include<stdlib.h>#define maxElements 4 // Unrolled Linked List Nodestruct Node{    int numElements;    int array[maxElements];    struct Node *next;}; /* Function to traverse an unrolled linked list   and print all the elements*/void printUnrolledList(struct Node *n){    while (n != NULL)    {        // Print elements in current node        for (int i=0; i<n->numElements; i++)            printf("%d ", n->array[i]);         // Move to next node        n = n->next;    }} // Program to create an unrolled linked list// with 3 Nodesint main(){    struct Node* head = NULL;    struct Node* second = NULL;    struct Node* third = NULL;     // allocate 3 Nodes    head = (struct Node*)malloc(sizeof(struct Node));    second = (struct Node*)malloc(sizeof(struct Node));    third = (struct Node*)malloc(sizeof(struct Node));     // Let us put some values in second node (Number    // of values must be less than or equal to    // maxElement)    head->numElements = 3;    head->array[0] = 1;    head->array[1] = 2;    head->array[2] = 3;     // Link first Node with the second Node    head->next = second;     // Let us put some values in second node (Number    // of values must be less than or equal to    // maxElement)    second->numElements = 3;    second->array[0] = 4;    second->array[1] = 5;    second->array[2] = 6;     // Link second Node with the third Node    second->next = third;     // Let us put some values in third node (Number    // of values must be less than or equal to    // maxElement)    third->numElements = 3;    third->array[0] = 7;    third->array[1] = 8;    third->array[2] = 9;    third->next = NULL;     printUnrolledList(head);     return 0;}
// Java program to implement unrolled// linked list and traversing it.import java.util.*; class GFG{     static final int maxElements = 4; // Unrolled Linked List Nodestatic class Node{    int numElements;    int []array = new int[maxElements];    Node next;}; // Function to traverse an unrolled// linked list  and print all the elementsstatic void printUnrolledList(Node n){    while (n != null)    {               // Print elements in current node        for(int i = 0; i < n.numElements; i++)            System.out.print(n.array[i] + " ");         // Move to next node        n = n.next;    }} // Program to create an unrolled linked list// with 3 Nodespublic static void main(String[] args){    Node head = null;    Node second = null;    Node third = null;     // Allocate 3 Nodes    head = new Node();    second = new Node();    third = new Node();     // Let us put some values in second    // node (Number of values must be    // less than or equal to maxElement)    head.numElements = 3;    head.array[0] = 1;    head.array[1] = 2;    head.array[2] = 3;     // Link first Node with the    // second Node    head.next = second;     // Let us put some values in    // second node (Number of values    // must be less than or equal to    // maxElement)    second.numElements = 3;    second.array[0] = 4;    second.array[1] = 5;    second.array[2] = 6;     // Link second Node with the third Node    second.next = third;     // Let us put some values in third    // node (Number of values must be    // less than or equal to maxElement)    third.numElements = 3;    third.array[0] = 7;    third.array[1] = 8;    third.array[2] = 9;    third.next = null;     printUnrolledList(head);}} // This code is contributed by amal kumar choubey 
# Python3 program to implement unrolled# linked list and traversing it.maxElements = 4 # Unrolled Linked List Nodeclass Node:         def __init__(self):                 self.numElements = 0        self.array = [0 for i in range(maxElements)]        self.next = None # Function to traverse an unrolled linked list# and print all the elementsdef printUnrolledList(n):     while (n != None):         # Print elements in current node        for i in range(n.numElements):            print(n.array[i], end = ' ')         # Move to next node        n = n.next # Driver Codeif __name__=='__main__':         head = None    second = None    third = None     # Allocate 3 Nodes    head = Node()    second = Node()    third = Node()     # Let us put some values in second    # node (Number of values must be    # less than or equal to    # maxElement)    head.numElements = 3    head.array[0] = 1    head.array[1] = 2    head.array[2] = 3     # Link first Node with the second Node    head.next = second     # Let us put some values in second node    # (Number of values must be less than    # or equal to maxElement)    second.numElements = 3    second.array[0] = 4    second.array[1] = 5    second.array[2] = 6     # Link second Node with the third Node    second.next = third     # Let us put some values in third node    # (Number of values must be less than    # or equal to maxElement)    third.numElements = 3    third.array[0] = 7    third.array[1] = 8    third.array[2] = 9    third.next = None     printUnrolledList(head)     # This code is contributed by rutvik_56
// C# program to implement unrolled// linked list and traversing it.using System;class GFG{     static readonly int maxElements = 4; // Unrolled Linked List Nodeclass Node{  public int numElements;  public int []array = new int[maxElements];  public Node next;}; // Function to traverse an unrolled// linked list  and print all the elementsstatic void printUnrolledList(Node n){  while (n != null)  {    // Print elements in current node    for(int i = 0; i < n.numElements; i++)      Console.Write(n.array[i] + " ");     // Move to next node    n = n.next;  }} // Program to create an unrolled linked list// with 3 Nodespublic static void Main(String[] args){  Node head = null;  Node second = null;  Node third = null;   // Allocate 3 Nodes  head = new Node();  second = new Node();  third = new Node();   // Let us put some values in second  // node (Number of values must be  // less than or equal to maxElement)  head.numElements = 3;  head.array[0] = 1;  head.array[1] = 2;  head.array[2] = 3;   // Link first Node with the  // second Node  head.next = second;   // Let us put some values in  // second node (Number of values  // must be less than or equal to  // maxElement)  second.numElements = 3;  second.array[0] = 4;  second.array[1] = 5;  second.array[2] = 6;   // Link second Node with the third Node  second.next = third;   // Let us put some values in third  // node (Number of values must be  // less than or equal to maxElement)  third.numElements = 3;  third.array[0] = 7;  third.array[1] = 8;  third.array[2] = 9;  third.next = null;   printUnrolledList(head);}} // This code is contributed by Rajput-Ji
<script>       // JavaScript program to implement unrolled      // linked list and traversing it.      const maxElements = 4;       // Unrolled Linked List Node      class Node {        constructor() {          this.numElements = 0;          this.array = new Array(maxElements);          this.next = null;        }      }       // Function to traverse an unrolled      // linked list and print all the elements      function printUnrolledList(n) {        while (n != null) {          // Print elements in current node          for (var i = 0; i < n.numElements; i++)            document.write(n.array[i] + " ");           // Move to next node          n = n.next;        }      }       // Program to create an unrolled linked list      // with 3 Nodes       var head = null;      var second = null;      var third = null;       // Allocate 3 Nodes      head = new Node();      second = new Node();      third = new Node();       // Let us put some values in second      // node (Number of values must be      // less than or equal to maxElement)      head.numElements = 3;      head.array[0] = 1;      head.array[1] = 2;      head.array[2] = 3;       // Link first Node with the      // second Node      head.next = second;       // Let us put some values in      // second node (Number of values      // must be less than or equal to      // maxElement)      second.numElements = 3;      second.array[0] = 4;      second.array[1] = 5;      second.array[2] = 6;       // Link second Node with the third Node      second.next = third;       // Let us put some values in third      // node (Number of values must be      // less than or equal to maxElement)      third.numElements = 3;      third.array[0] = 7;      third.array[1] = 8;      third.array[2] = 9;      third.next = null;       printUnrolledList(head);       </script>
1 2 3 4 5 6 7 8 9 
Complexity Analysis:
Time Complexity: O(n).
Space Complexity: O(n).
In this article, we have introduced an unrolled list and advantages of it. We have also shown how to traverse the list. In the next article, we will be discussing the insertion, deletion, and values of maxElements/numElements in detail.
Insertion in Unrolled Linked List
This article is contributed by Harsh Agarwal. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. 
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