Union and Intersection of two Linked Lists
Last Updated :
23 Jul, 2025
Given two singly Linked Lists, create union and intersection lists that contain the union and intersection of the elements present in the given lists. Each of the two linked lists contains distinct node values.
Note: The order of elements in output lists doesn’t matter.
Example:
Input:
head1: 10 -> 15 -> 4 -> 20
head2: 8 -> 4 -> 2 -> 10
Output:
Union: 10 -> 15 -> 4 -> 20 -> 8 -> 2
Intersection: 10 -> 4
Explanation: In these two lists 4 and 10 nodes are common. The union lists contain all the unique nodes of both lists.
Input:
head1 : 1 -> 2 -> 3 -> 4
head2 : 3 -> 4 -> 8 -> 10
Output:
Union: 1 -> 2 -> 3 -> 4 -> 8 -> 10
Intersection: 3 -> 4
Explanation: In these two lists 4 and 3 nodes are common. The union lists contain all the unique nodes of both lists.
[Naive Approach] Using Two Nested Loops - O(n * m) Time and O(n + m) Space:
The idea is to traverse both linked lists and check each element's presence in the result list to avoid duplicates. For the union, we add all unique elements from both lists. For the intersection, we only add elements that are present in both lists.
- For union - We traverse both linked lists one by one, and for each element check if the element is already present in the resulting union list, If the element is not present, insert it into the union list.
- For Intersection - We traverse the first linked list, and for each element check if the element is present in the second linked list and not already in the resulting intersection list then we will insert it into the intersection list.
Below is the implementation of the above approach:
C++
// C++ program to find union and intersection of two
// unsorted linked lists
#include <iostream>
using namespace std;
class Node {
public:
int data;
Node* next;
Node(int x) {
data = x;
next = nullptr;
}
};
// Function to check if a value is present in the linked list
bool isPresent(Node* head, int value) {
Node* curr = head;
while (curr != nullptr) {
if (curr->data == value) {
return true;
}
curr = curr->next;
}
return false;
}
// Function to find the union of two linked lists
Node* findUnion(Node* head1, Node* head2) {
Node* result = nullptr;
Node* tail = nullptr;
Node* curr1 = head1, *curr2 = head2;
// Insert all elements from the first list
// into the result list
while (curr1 != nullptr) {
if (!isPresent(result, curr1->data)) {
Node* newNode = new Node(curr1->data);
if (result == nullptr) {
result = newNode;
tail = result;
} else {
tail->next = newNode;
tail = newNode;
}
}
curr1 = curr1->next;
}
// Insert elements from the second list into the
// result list if they are not already present
while (curr2 != nullptr) {
if (!isPresent(result, curr2->data)) {
Node* newNode = new Node(curr2->data);
if (result == nullptr) {
result = newNode;
tail = result;
} else {
tail->next = newNode;
tail = newNode;
}
}
curr2 = curr2->next;
}
return result;
}
// Function to find the intersection of two linked
// lists using brute force
Node* findIntersection(Node* head1, Node* head2) {
Node* result = nullptr;
Node* tail = nullptr;
Node* curr1 = head1, *curr2 = head2;
// For each element in the first list, check if
// it is present in the second list
while (curr1 != nullptr) {
if (isPresent(curr2, curr1->data)
&& !isPresent(result, curr1->data)) {
Node* newNode = new Node(curr1->data);
if (result == nullptr) {
result = newNode;
tail = result;
} else {
tail->next = newNode;
tail = newNode;
}
}
curr1 = curr1->next;
}
return result;
}
void printList(Node* head) {
Node* curr = head;
while (curr != nullptr) {
cout << curr->data << " ";
curr = curr->next;
}
cout << endl;
}
int main() {
// Creating two hardcoded linked lists
Node* head1 = nullptr;
Node* head2 = nullptr;
// Insert elements into the first linked list
// 10 -> 15 -> 4 -> 20
head1 = new Node(10);
head1->next = new Node(15);
head1->next->next = new Node(4);
head1->next->next->next = new Node(20);
// Insert elements into the second linked list
// 8 -> 4 -> 2 -> 10
head2 = new Node(8);
head2->next = new Node(4);
head2->next->next = new Node(2);
head2->next->next->next = new Node(10);
Node* unionList = findUnion(head1, head2);
Node* intersectionList = findIntersection(head1, head2);
cout << "Union: ";
printList(unionList);
cout << "Intersection: ";
printList(intersectionList);
return 0;
}
// C program to find union and intersection of two
// unsorted linked lists
#include <stdio.h>
#include <stdlib.h>
struct Node {
int data;
struct Node* next;
};
struct Node* createNode(int x);
// Function to check if a value is present in the linked list
int isPresent(struct Node* head, int value) {
struct Node* curr = head;
while (curr != NULL) {
if (curr->data == value) {
return 1;
}
curr = curr->next;
}
return 0;
}
// Function to find the union of two linked lists
struct Node* findUnion(struct Node* head1, struct Node* head2) {
struct Node* result = NULL;
struct Node* tail = NULL;
struct Node* curr1 = head1;
struct Node* curr2 = head2;
// Insert all elements from the first list into the result list
while (curr1 != NULL) {
if (!isPresent(result, curr1->data)) {
struct Node* newNode = createNode(curr1->data);
if (result == NULL) {
result = newNode;
tail = result;
} else {
tail->next = newNode;
tail = newNode;
}
}
curr1 = curr1->next;
}
// Insert elements from the second list into the
// result list if they are not already present
while (curr2 != NULL) {
if (!isPresent(result, curr2->data)) {
struct Node* newNode = createNode(curr2->data);
if (result == NULL) {
result = newNode;
tail = result;
} else {
tail->next = newNode;
tail = newNode;
}
}
curr2 = curr2->next;
}
return result;
}
// Function to find the intersection of two linked
// lists using brute force
struct Node* findIntersection(struct Node* head1, struct Node* head2) {
struct Node* result = NULL;
struct Node* tail = NULL;
struct Node* curr1 = head1;
struct Node* curr2 = head2;
// For each element in the first list, check if it is
// present in the second list
while (curr1 != NULL) {
if (isPresent(curr2, curr1->data)
&& !isPresent(result, curr1->data)) {
struct Node* newNode = createNode(curr1->data);
if (result == NULL) {
result = newNode;
tail = result;
} else {
tail->next = newNode;
tail = newNode;
}
}
curr1 = curr1->next;
}
return result;
}
void printList(struct Node* head) {
struct Node* curr = head;
while (curr != NULL) {
printf("%d ", curr->data);
curr = curr->next;
}
printf("\n");
}
struct Node* createNode(int x) {
struct Node* newNode =
(struct Node*)malloc(sizeof(struct Node));
newNode->data = x;
newNode->next = NULL;
return newNode;
}
int main() {
// Creating two hardcoded linked lists
// Insert elements into the first linked list
// 10 -> 15 -> 4 -> 20
struct Node* head1 = createNode(10);
head1->next = createNode(15);
head1->next->next = createNode(4);
head1->next->next->next = createNode(20);
// Insert elements into the second linked list
// 8 -> 4 -> 2 -> 10
struct Node* head2 = createNode(8);
head2->next = createNode(4);
head2->next->next = createNode(2);
head2->next->next->next = createNode(10);
struct Node* unionList = findUnion(head1, head2);
struct Node* intersectionList = findIntersection(head1, head2);
printf("Union: ");
printList(unionList);
printf("Intersection: ");
printList(intersectionList);
return 0;
}
// Java program to find union and intersection of two
// unsorted linked lists
class Node {
int data;
Node next;
Node(int x) {
data = x;
next = null;
}
}
class GfG {
// Function to check if a value is present in the linked list
static boolean isPresent(Node head, int value) {
Node curr = head;
while (curr != null) {
if (curr.data == value) {
return true;
}
curr = curr.next;
}
return false;
}
// Function to find the union of two linked lists
static Node findUnion(Node head1, Node head2) {
Node result = null;
Node tail = null;
Node curr1 = head1, curr2 = head2;
// Insert all elements from the first
// list into the result list
while (curr1 != null) {
if (!isPresent(result, curr1.data)) {
Node newNode = new Node(curr1.data);
if (result == null) {
result = newNode;
tail = result;
} else {
tail.next = newNode;
tail = newNode;
}
}
curr1 = curr1.next;
}
// Insert elements from the second list into the
// result list if they are not already present
while (curr2 != null) {
if (!isPresent(result, curr2.data)) {
Node newNode = new Node(curr2.data);
if (result == null) {
result = newNode;
tail = result;
} else {
tail.next = newNode;
tail = newNode;
}
}
curr2 = curr2.next;
}
return result;
}
// Function to find the intersection of two linked
// lists using brute force
static Node findIntersection(Node head1, Node head2) {
Node result = null;
Node tail = null;
Node curr1 = head1, curr2 = head2;
// For each element in the first list, check if it
// is present in the second list
while (curr1 != null) {
if (isPresent(curr2, curr1.data)
&& !isPresent(result, curr1.data)) {
Node newNode = new Node(curr1.data);
if (result == null) {
result = newNode;
tail = result;
} else {
tail.next = newNode;
tail = newNode;
}
}
curr1 = curr1.next;
}
return result;
}
static void printList(Node head) {
Node curr = head;
while (curr != null) {
System.out.print(curr.data + " ");
curr = curr.next;
}
System.out.println();
}
public static void main(String[] args) {
// Insert elements into the first linked list
// 10 -> 15 -> 4 -> 20
Node head1 = new Node(10);
head1.next = new Node(15);
head1.next.next = new Node(4);
head1.next.next.next = new Node(20);
// Insert elements into the second linked list
// 8 -> 4 -> 2 -> 10
Node head2 = new Node(8);
head2.next = new Node(4);
head2.next.next = new Node(2);
head2.next.next.next = new Node(10);
Node unionList = findUnion(head1, head2);
Node intersectionList = findIntersection(head1, head2);
System.out.print("Union: ");
printList(unionList);
System.out.print("Intersection: ");
printList(intersectionList);
}
}
# Python program to find union and intersection of two
# unsorted linked lists
class Node:
def __init__(self, x):
self.data = x
self.next = None
# Function to check if a value is present in the linked list
def isPresent(head, value):
curr = head
while curr is not None:
if curr.data == value:
return True
curr = curr.next
return False
# Function to find the union of two linked lists
def findUnion(head1, head2):
result = None
tail = None
curr1, curr2 = head1, head2
# Insert all elements from the first list into the result list
while curr1 is not None:
if not isPresent(result, curr1.data):
newNode = Node(curr1.data)
if result is None:
result = newNode
tail = result
else:
tail.next = newNode
tail = newNode
curr1 = curr1.next
# Insert elements from the second list into the
# result list if they are not already present
while curr2 is not None:
if not isPresent(result, curr2.data):
newNode = Node(curr2.data)
if result is None:
result = newNode
tail = result
else:
tail.next = newNode
tail = newNode
curr2 = curr2.next
return result
# Function to find the intersection of two
# linked lists using brute force
def findIntersection(head1, head2):
result = None
tail = None
curr1, curr2 = head1, head2
# For each element in the first list,
# check if it is present in the second list
while curr1 is not None:
if isPresent(curr2, curr1.data) \
and not isPresent(result, curr1.data):
newNode = Node(curr1.data)
if result is None:
result = newNode
tail = result
else:
tail.next = newNode
tail = newNode
curr1 = curr1.next
return result
def printList(head):
curr = head
while curr is not None:
print(curr.data, end=" ")
curr = curr.next
print()
if __name__ == "__main__":
# Creating two hardcoded linked lists
# Insert elements into the first linked list
# 10 -> 15 -> 4 -> 20
head1 = Node(10)
head1.next = Node(15)
head1.next.next = Node(4)
head1.next.next.next = Node(20)
# Insert elements into the second linked list
# 8 -> 4 -> 2 -> 10
head2 = Node(8)
head2.next = Node(4)
head2.next.next = Node(2)
head2.next.next.next = Node(10)
unionList = findUnion(head1, head2)
intersectionList = findIntersection(head1, head2)
print("Union: ", end="")
printList(unionList)
print("Intersection: ", end="")
printList(intersectionList)
// C# program to find union and intersection of two
// unsorted linked lists
using System;
class Node {
public int data;
public Node next;
public Node(int x) {
data = x;
next = null;
}
}
class GfG {
// Function to check if a value is present in the linked list
static bool IsPresent(Node head, int value) {
Node curr = head;
while (curr != null) {
if (curr.data == value) {
return true;
}
curr = curr.next;
}
return false;
}
// Function to find the union of two linked lists
static Node FindUnion(Node head1, Node head2) {
Node result = null;
Node tail = null;
Node curr1 = head1, curr2 = head2;
// Insert all elements from the first list into the result list
while (curr1 != null) {
if (!IsPresent(result, curr1.data)) {
Node newNode = new Node(curr1.data);
if (result == null) {
result = newNode;
tail = result;
} else {
tail.next = newNode;
tail = newNode;
}
}
curr1 = curr1.next;
}
// Insert elements from the second list into the
// result list if they are not already present
while (curr2 != null) {
if (!IsPresent(result, curr2.data)) {
Node newNode = new Node(curr2.data);
if (result == null) {
result = newNode;
tail = result;
} else {
tail.next = newNode;
tail = newNode;
}
}
curr2 = curr2.next;
}
return result;
}
// Function to find the intersection of two
// linked lists using brute force
static Node FindIntersection(Node head1, Node head2) {
Node result = null;
Node tail = null;
Node curr1 = head1, curr2 = head2;
// For each element in the first list, check
//if it is present in the second list
while (curr1 != null) {
if (IsPresent(curr2, curr1.data)
&& !IsPresent(result, curr1.data)) {
Node newNode = new Node(curr1.data);
if (result == null) {
result = newNode;
tail = result;
} else {
tail.next = newNode;
tail = newNode;
}
}
curr1 = curr1.next;
}
return result;
}
static void PrintList(Node head) {
Node curr = head;
while (curr != null) {
Console.Write(curr.data + " ");
curr = curr.next;
}
Console.WriteLine();
}
static void Main(string[] args) {
// Creating two hardcoded linked lists
// Insert elements into the first linked list
// 10 -> 15 -> 4 -> 20
Node head1 = new Node(10);
head1.next = new Node(15);
head1.next.next = new Node(4);
head1.next.next.next = new Node(20);
// Insert elements into the second linked list
// 8 -> 4 -> 2 -> 10
Node head2 = new Node(8);
head2.next = new Node(4);
head2.next.next = new Node(2);
head2.next.next.next = new Node(10);
Node unionList = FindUnion(head1, head2);
Node intersectionList = FindIntersection(head1, head2);
Console.Write("Union: ");
PrintList(unionList);
Console.Write("Intersection: ");
PrintList(intersectionList);
}
}
// Javascript program to find union and intersection of two
// unsorted linked lists
class Node {
constructor(x) {
this.data = x;
this.next = null;
}
}
// Function to check if a value is present in the linked
// list
function isPresent(head, value) {
let curr = head;
while (curr !== null) {
if (curr.data === value) {
return true;
}
curr = curr.next;
}
return false;
}
// Function to find the union of two linked lists
function findUnion(head1, head2) {
let result = null;
let tail = null;
let curr1 = head1, curr2 = head2;
// Insert all elements from the first list into the
// result list
while (curr1 !== null) {
if (!isPresent(result, curr1.data)) {
let newNode = new Node(curr1.data);
if (result === null) {
result = newNode;
tail = result;
}
else {
tail.next = newNode;
tail = newNode;
}
}
curr1 = curr1.next;
}
// Insert elements from the second list into the
// result list if they are not already present
while (curr2 !== null) {
if (!isPresent(result, curr2.data)) {
let newNode = new Node(curr2.data);
if (result === null) {
result = newNode;
tail = result;
}
else {
tail.next = newNode;
tail = newNode;
}
}
curr2 = curr2.next;
}
return result;
}
// Function to find the intersection of two linked lists
// using brute force
function findIntersection(head1, head2) {
let result = null;
let tail = null;
let curr1 = head1, curr2 = head2;
// For each element in the first list, check if it is
// present in the second list
while (curr1 !== null) {
if (isPresent(curr2, curr1.data)
&& !isPresent(result, curr1.data)) {
let newNode = new Node(curr1.data);
if (result === null) {
result = newNode;
tail = result;
}
else {
tail.next = newNode;
tail = newNode;
}
}
curr1 = curr1.next;
}
return result;
}
function printList(head) {
let curr = head;
while (curr !== null) {
console.log(curr.data + " ");
curr = curr.next;
}
console.log();
}
// Insert elements into the first linked list
// 10 -> 15 -> 4 -> 20
let head1 = new Node(10);
head1.next = new Node(15);
head1.next.next = new Node(4);
head1.next.next.next = new Node(20);
// Insert elements into the second linked list
// 8 -> 4 -> 2 -> 10
let head2 = new Node(8);
head2.next = new Node(4);
head2.next.next = new Node(2);
head2.next.next.next = new Node(10);
let unionList = findUnion(head1, head2);
let intersectionList = findIntersection(head1, head2);
console.log("Union: ");
printList(unionList);
console.log("Intersection: ");
printList(intersectionList);
OutputUnion: 10 15 4 20 8 2
Intersection: 10 4
Time Complexity: O(m * n) , where m and n are the length of the two Linked Lists.
Space Complexity: O(m + n)
[Optimal Approach] Using Sorting - O(nLogn + mLogm) Time and O(n + m) Space:
The idea is to sort the given lists using merge sort, then we linearly search both sorted lists to obtain the union and intersection. By Keeping two pointers (initially pointing to the first node of the respective lists) compare the node values :
- If the values are equal, add the value to both the union and intersection lists, then move both pointers to the next node.
- else if the values are not equal, insert the smaller value into the union list and move the corresponding pointer to the next node.
- If one of the pointers becomes null, traverse the remaining nodes of the other list and add them to the union list.
Please refer to Union and Intersection of Two Linked Lists Using Merge Sort for Implemenation.
[Expected Approach] Using Hashing – O(n + m) Time and O(n + m) Space:
The idea is to use HashSet to efficiently determine the union and intersection of two linked lists.
- For Intersection - By storing the elements of one list in a hash set, we can quickly check for the presence of elements from the second list to get Intersection.
- For Union - By storing the elements of both list in a hash set, we can get unique elements from both lists to get union.
Please refer to Union and Intersection of Two Linked Lists Using Hashing for Implemenation.
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