Union and Intersection of two linked lists | Set-3 (Hashing)

Given two Linked Lists, create union and intersection lists that contain union and intersection of the elements present in the given lists. Order of elements in output lists doesn’t matter.


   List1: 10 -> 15 -> 4 -> 20
   list2:  8 -> 4 -> 2 -> 10
   Intersection List: 4 -> 10
   Union List: 2 -> 8 -> 20 -> 4 -> 15 -> 10

We have already discussed Method-1 and Method-2 of this question.
In this post, its Method-3 (Using Hashing) is discussed with a Time Complexity of O(m+n) i.e. better than both methods discussed earlier.

1- Start traversing both the lists.
   a) Store the current element of both lists
      with its occurrence in the map.
2- For Union: Store all the elements of the map 
   in the resultant list.
3- For Intersection: Store all the elements only 
   with an occurrence of 2 as 2 denotes that 
   they are present in both the lists.

Below is the C++ implementation of the above steps.





// C++ program to find union and intersection of
// two unsorted linked lists in O(m+n) time.
#include <bits/stdc++.h>
using namespace std;
/* Link list node */
struct Node {
    int data;
    struct Node* next;
/* A utility function to insert a node at the
   beginning of a linked list*/
void push(struct Node** head_ref, int new_data)
    /* allocate node */
    struct Node* new_node = (struct Node*)malloc(
        sizeof(struct 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;
/* Utility function to store the 
   elements of both list */
void storeEle(struct Node* head1, struct Node* head2,
              unordered_map<int, int>& eleOcc)
    struct Node* ptr1 = head1;
    struct Node* ptr2 = head2;
    // Traverse both lists
    while (ptr1 != NULL || ptr2 != NULL) {
        // store element in the map
        if (ptr1 != NULL) {
            ptr1 = ptr1->next;
        // store element in the map
        if (ptr2 != NULL) {
            ptr2 = ptr2->next;
/* Function to get the union of two 
   linked lists head1 and head2 */
struct Node* getUnion(
    unordered_map<int, int> eleOcc)
    struct Node* result = NULL;
    // Push all the elements into
    // the resultant list
    for (auto it = eleOcc.begin(); it != eleOcc.end(); it++)
        push(&result, it->first);
    return result;
/* Function to get the intersection of 
   two linked lists head1 and head2 */
struct Node* getIntersection(
    unordered_map<int, int> eleOcc)
    struct Node* result = NULL;
    // Push a node with an element
    // having occurrence of 2 as that
    // means the current element is
    // present in both the lists
    for (auto it = eleOcc.begin();
         it != eleOcc.end(); it++)
        if (it->second == 2)
            push(&result, it->first);
    // return resultant list
    return result;
/* A utility function to print a linked list*/
void printList(struct Node* node)
    while (node != NULL) {
        printf("%d ", node->data);
        node = node->next;
// Prints union and intersection of
// lists with head1 and head2.
void printUnionIntersection(Node* head1,
                            Node* head2)
    // Store all the elements of
    // both lists in the map
    unordered_map<int, int> eleOcc;
    storeEle(head1, head2, eleOcc);
    Node* intersection_list = getIntersection(eleOcc);
    Node* union_list = getUnion(eleOcc);
    printf("\nIntersection list is \n");
    printf("\nUnion list is \n");
/* Driver program to test above function*/
int main()
    /* Start with the empty list */
    struct Node* head1 = NULL;
    struct Node* head2 = NULL;
    /* create a linked list 11->10->15->4->20 */
    push(&head1, 1);
    push(&head1, 2);
    push(&head1, 3);
    push(&head1, 4);
    push(&head1, 5);
    /* create a linked list 8->4->2->10 */
    push(&head2, 1);
    push(&head2, 3);
    push(&head2, 5);
    push(&head2, 6);
    printf("First list is \n");
    printf("\nSecond list is \n");
    printUnionIntersection(head1, head2);
    return 0;



First list is 
5 4 3 2 1 
Second list is 
6 5 3 1 
Intersection list is 
3 5 1 
Union list is 
3 4 6 5 2 1 

We can also handle the case of duplicates by maintaining separate Hash for both the lists.

Complexity Analysis:

  • Time Complexity: O(m+n).
    Here ‘m’ and ‘n’ are number of elements present in first and second lists respectively.
    For Union: Traverse both the lists, store the elements in Hash-map and update the respective count.
    For Intersection: Check if count of an element in hash-map is ‘2’.
  • Auxiliary Space: O(m+n).
    Use of Hash-map data structure for storing values.

This article is contributed by Sahil Chhabra. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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Improved By : nidhi_biet, bidibaaz123