Sort the linked list in the order of elements appearing in the array

Given an array of size N and a Linked List where elements will be from the array but can also be duplicated, sort the linked list in the order, elements are appearing in the array. It may be assumed that the array covers all elements of the linked list.

arr[] =

list =

Sorted list =

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First, make a hash table that stores the frequencies of elements in linked list. Then, simply traverse list and for each element of arr[i] check the frequency in the has table and modify the data of list by arr[i] element upto its frequency and at last Print the list.

// Efficient CPP program to sort given list in order
// elements are appearing in an array
#include <bits/stdc++.h>
using namespace std;
  
// Linked list node
struct Node {
    int data;
    struct Node* next;
};
  
// function prototype for printing the list
void printList(struct Node*);
  
// Function to insert a node at the
// beginning of the linked list
void push(struct Node** head_ref, int new_data)
{
    struct Node* new_node = new Node;
    new_node -> data = new_data;
    new_node -> next = *head_ref;
    *head_ref = new_node;
}
  
// function to print the linked list
void printList(struct Node* head)
{
    while (head != NULL) {
        cout << head -> data << " -> ";
        head = head -> next;
    }
}
  
// Function that sort list in order of apperaing
// elements in an array
void sortlist(int arr[], int N, struct Node* head)
{
    // Store frequencies of elements in a
    // hash table.
    unordered_map<int, int> hash;
    struct Node* temp = head;
    while (temp) {        
        hash[temp -> data]++;
        temp = temp -> next;
    }
  
    temp = head;
   
    // One by one put elements in lis according
    // to their appearance in array
    for (int i = 0; i < N; i++) {        
  
        // Update 'frequency' nodes with value 
        // equal to arr[i]
        int frequency = hash[arr[i]];
        while (frequency--) {
  
            // Modify list data as element 
            // appear in an array
            temp -> data = arr[i];
            temp = temp -> next;
        }
    }
}
  
// Driver Code
int main()
{
    struct Node* head = NULL;
    int arr[] = { 5, 1, 3, 2, 8 };
    int N = sizeof(arr) / sizeof(arr[0]);
  
    // creating the linked list
    push(&head, 3);
    push(&head, 2);
    push(&head, 5);
    push(&head, 8);
    push(&head, 5);
    push(&head, 2);
    push(&head, 1);
  
    // Function call to sort the list in order
    // elements are apperaing in an array
    sortlist(arr, N, head);
  
    // print the modified linked list
    cout << "Sorted List:" << endl;
    printList(head);
    return 0;
}

Output :

Sort list:
5 -> 5 -> 1 -> 3 -> 2 -> 2 -> 8 


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