Implement union and intersection of two sorted linked lists

Here, we are going to learn how to implement union and intersection of two sorted linked lists using C program?
Submitted by Indrajeet Das, on November 13, 2018

In computer science, a linked list is a linear collection of data elements, whose order is not given by their physical placement in memory. Instead, each element points to the next. It is a data structure consisting of a collection of nodes which together represent a sequence. In its most basic form, each node contains data and a reference (in other words, a link) to the next node in the sequence. This structure allows for efficient insertion or removal of elements from any position in the sequence during iteration. More complex variants add additional links, allowing more efficient insertion or removal of nodes at arbitrary positions.

Union of two linked lists can be found by using merging the lists in a sorted manner.

The intersection of the two lists can be found by only taking common elements while merging the two lists.

It has been assumed that the linked lists are sorted.

Algorithm:

Union(L1,L2)

    1) Declare node pointer output, output Tail as NULL
    2) Repeat steps 3 to 9 while L1!=NULL AND L2!=NULL
    3) Make a newNode and set its next = NULL
    4) If L1->data < L2->data then
        Set newNode->data = L1->data
        Set L1 = L1->next
    5) Else if L1->data > L2->data then
        Set newNode->data = L2->data
        L2 = L2->next
    6) Else
        i) Set Data = L1->data
        ii) Set newNode->data = Data
        iii) Repeat steps a) and b) 
        while L1!=NULL AND L2!=NULL AND L1->data == Data AND L2->data == Data
            a) Set L1 = L1->next
            b) Set L2 = L2->next

    7) If output == NULL then
        Set Output = outputTail = newNode

    8) Else
        a) Set outputTail->next = newNode
        b) Set outputTail = outputTail->next
     9) Repeat steps 10 to 14 while L1!=NULL
    10) Make a newNode
    11) Set outputTail->next = newNode
    12) Set outputTail = outputTail->next
    13) Set outputTail->data = L1->data
    14) Set L1 = L1->next
        Repeat steps 15 to 19 while L2!=NULL
    15) Make a newNode
    16) Set outputTail->next = newNode
    17) Set outputTail = outputTail->next
    18) Set outputTail->data = L2->data
    19) Set L2 = L2->next
        Return output

Intersection(L1,L2)

    1.	If L1 or L2 is NULL then return NULL
    2.	Declare node pointers output, outputTail as null.
    3.	Repeat steps 4 to 6 while L1!=NULL AND L2!=NULL
    4.	If L1->datadata then
        Set L1 = L1->next
    5.	Else If L2->datadata then
        Set L2 = L2->next
    6.	Else
        a)	Declare and set data = L1->data
        b)	Make a newNode
        c)	Set newNode->data = data and newNode->next = NULL
        d)	If output == null then
            i.	Set output = outputTail = newNode
        e)	Else
            i.	Set outputTail->next = newNode
            ii.	Set outputTail = outputTail->next
        f)	Repeat steps i and ii 
        while L1!=NULL AND L2!=NULL AND L1->data == data AND L2->data == data
            i.	Set L1 = L1->next
            ii.	Set L2 = L2->next

Code:

#include <stdio.h>
#include <stdlib.h>

struct node{
	struct node*next;
	int data;
};

struct node * Union(struct node * L1, struct node * L2){
	struct node * output = NULL;
	struct node * outTail = NULL;
	while(L1&&L2){
		struct node * newNode = (struct node *) malloc(sizeof(struct node));
		newNode->next = NULL;
		if(L1->data<L2->data){
			newNode->data = L1->data;
			L1 = L1->next;
		}
		else if(L1->data>L2->data){
			newNode->data = L2->data;
			L2 = L2->next;
		}
		else{
			int data = L1->data;
			newNode->data = data;
			while(L1 && L2 && L1->data == data && L2->data == data){
				L1 = L1->next;
				L2 = L2->next;
			}
		}
		if(!output)
			output = outTail = newNode;
		else{
			outTail->next = newNode;
			outTail = outTail->next;
		}
	}
	while(L1){
		outTail->next = (struct node *) malloc(sizeof(struct node));
		outTail = outTail->next;
		outTail->data = L1->data;
		L1 = L1->next;
	}
	while(L2){
		outTail->next = (struct node *) malloc(sizeof(struct node));
		outTail = outTail->next;
		outTail->data = L2->data;
		L2 = L2->next;
	}
	outTail->next = NULL;
	return output;
}

struct node * intersection(struct node * L1, struct node* L2){
	if(L1 == NULL || L2 == NULL)
		return NULL;
	struct node * output = NULL;
	struct node * outTail = NULL;
	while(L1&&L2){
		if(L1->data<L2->data){
			L1 = L1->next;
		}
		else if(L2->data<L1->data){
			L2 = L2->next;
		}
		else{
			int data = L1->data;
			struct node * newNode = (struct node *) malloc(sizeof(struct node));
			newNode->data = data;
			newNode->next = NULL;
			if(output == NULL){
				outTail = output = newNode;
			}
			else{
				outTail->next = newNode;
				outTail = outTail->next;
			}
			while(L1 && L2 && L1->data == data && L2->data == data){
				L1 = L1->next;
				L2 = L2->next;
			}
		}
	}
	return output;
}

struct node * createList(int listNum){
	struct node * list = NULL;
	struct node * list_tail = NULL;
	printf("Enter elements of List %d in increasing order\n",listNum);
	char ch = 'y';
	do{
		int data;
		printf("Enter element : ");
		scanf("%d",&data);
		struct node * newNode = (struct node *) malloc(sizeof(struct node));
		newNode->data = data;
		newNode->next = NULL;
		if(list == NULL){
			list = list_tail = newNode;
		}
		else{
			list_tail->next = newNode;
			list_tail = list_tail->next;
		}
		printf("Would you like to insert another element [Y/N] : ");
		scanf(" %c",&ch);
	}while(ch == 'y' || ch == 'Y');
	
	return list;
}

void print(struct node * list){
	if(list == NULL){
		printf("Empty List\n");
		return;
	}
	while(list!=NULL){
		printf("%d ",list->data);
		list = list->next;
	}
	printf("\n");
}

int main() {
	struct node * L1 = NULL;
	struct node * L2 = NULL;
	struct node * L3 = NULL;
	struct node * L4 = NULL;
	L1 = createList(1);
	L2 = createList(2);
	printf("List 1 : ");
	print(L1);
	printf("List 2 : ");
	print(L2);
	printf("Union : ");
	L3 = Union(L1, L2);
	print(L3);
	printf("Intersection : ");
	L4 = intersection(L1, L2);
	print(L4);
	return 0;
}

Output

Enter elements of List 1 in increasing order 
Enter element : 1
Would you like to insert another element [Y/N] : Y 
Enter element : 4
Would you like to insert another element [Y/N] : Y 
Enter element : 9
Would you like to insert another element [Y/N] : Y 
Enter element : 27 
Would you like to insert another element [Y/N] : N 
Enter elements of List 2 in increasing order 
Enter element : 4
Would you like to insert another element [Y/N] : Y 
Enter element : 9
Would you like to insert another element [Y/N] : Y 
Enter element : 18 
Would you like to insert another element [Y/N] : Y 
Enter element : 22 
Would you like to insert another element [Y/N] : Y 
Enter element : 30 
Would you like to insert another element [Y/N] : N 
List 1 : 1 4 9 27
List 2 : 4 9 18 22 30
Union : 1 4 9 18 22 27 30
Intersection : 4 9