Merge a linked list into another linked list at alternate positions

1.8

Given two linked lists, insert nodes of second list into first list at alternate positions of first list.
For example, if first list is 5->7->17->13->11 and second is 12->10->2->4->6, the first list should become 5->12->7->10->17->2->13->4->11->6 and second list should become empty. The nodes of second list should only be inserted when there are positions available. For example, if the first list is 1->2->3 and second list is 4->5->6->7->8, then first list should become 1->4->2->5->3->6 and second list to 7->8.

Use of extra space is not allowed (Not allowed to create additional nodes), i.e., insertion must be done in-place. Expected time complexity is O(n) where n is number of nodes in first list.

The idea is to run a loop while there are available positions in first loop and insert nodes of second list by changing pointers. Following are C and Java implementations of this approach.

C/C++

// C program to merge a linked list into another at
// alternate positions
#include <stdio.h>
#include <stdlib.h>

// A nexted list node
struct Node
{
    int data;
    struct Node *next;
};

/* Function to insert a node at the beginning */
void push(struct Node ** head_ref, int new_data)
{
    struct Node* new_node = 
           (struct Node*) malloc(sizeof(struct Node));
    new_node->data  = new_data;
    new_node->next = (*head_ref);
    (*head_ref)  = new_node;
}

/* Utility function to print a singly linked list */
void printList(struct Node *head)
{
    struct Node *temp = head;
    while (temp != NULL)
    {
        printf("%d ", temp->data);
        temp = temp->next;
    }
    printf("\n");
}

// Main function that inserts nodes of linked list q into p at 
// alternate positions. Since head of first list never changes 
// and head of second list  may change, we need single pointer
// for first list and double pointer for second list.
void merge(struct Node *p, struct Node **q)
{
     struct Node *p_curr = p, *q_curr = *q;
     struct Node *p_next, *q_next;

     // While therre are avialable positions in p
     while (p_curr != NULL && q_curr != NULL)
     {
         // Save next pointers
         p_next = p_curr->next;
         q_next = q_curr->next;

         // Make q_curr as next of p_curr
         q_curr->next = p_next;  // Change next pointer of q_curr
         p_curr->next = q_curr;  // Change next pointer of p_curr

         // Update current pointers for next iteration
         p_curr = p_next;
         q_curr = q_next;
    }

    *q = q_curr; // Update head pointer of second list
}

// Driver program to test above functions
int main()
{
     struct Node *p = NULL, *q = NULL;
     push(&p, 3);
     push(&p, 2);
     push(&p, 1);
     printf("First Linked List:\n");
     printList(p);

     push(&q, 8);
     push(&q, 7);
     push(&q, 6);
     push(&q, 5);
     push(&q, 4);
     printf("Second Linked List:\n");
     printList(q);

     merge(p, &q);

     printf("Modified First Linked List:\n");
     printList(p);

     printf("Modified Second Linked List:\n");
     printList(q);

     getchar();
     return 0;
}

Java

// Java program to merge a linked list into another at
// alternate positions
class LinkedList
{
    Node head;  // head of list

    /* Linked list Node*/
    class Node
    {
        int data;
        Node next;
        Node(int d) {data = d; next = null; }
    }

    /* Inserts a new Node at front of the list. */
    void push(int new_data)
    {
        /* 1 & 2: Allocate the Node &
                  Put in the data*/
        Node new_node = new Node(new_data);

        /* 3. Make next of new Node as head */
        new_node.next = head;

        /* 4. Move the head to point to new Node */
        head = new_node;
    }

    // Main function that inserts nodes of linked list q into p at
    // alternate positions. Since head of first list never changes
    // and head of second list/ may change, we need single pointer
    // for first list and double pointer for second list.
    void merge(LinkedList q)
    {
        Node p_curr = head, q_curr = q.head;
        Node p_next, q_next;

        // While there are available positions in p;
        while (p_curr != null && q_curr != null) {

            // Save next pointers
            p_next = p_curr.next;
            q_next = q_curr.next;

            // make q_curr as next of p_curr
            q_curr.next = p_next; // change next pointer of q_curr
            p_curr.next = q_curr; // change next pointer of p_curr

            // update current pointers for next iteration
            p_curr = p_next;
            q_curr = q_next;
        }
        q.head = q_curr;
    }

    /* Function to print linked list */
    void printList()
    {
        Node temp = head;
        while (temp != null)
        {
           System.out.print(temp.data+" ");
           temp = temp.next;
        }
        System.out.println();
    }

    /* Drier program to test above functions */
    public static void main(String args[])
    {
        LinkedList llist1 = new LinkedList();
        LinkedList llist2 = new LinkedList();
        llist1.push(3);
        llist1.push(2);
        llist1.push(1);

        System.out.println("First Linked List:");
        llist1.printList();

        llist2.push(8);
        llist2.push(7);
        llist2.push(6);
        llist2.push(5);
        llist2.push(4);

        System.out.println("Second Linked List:");

        llist1.merge(llist2);

        System.out.println("Modified first linked list:");
        llist1.printList();

        System.out.println("Modified second linked list:");
        llist2.printList();
    }
} /* This code is contributed by Rajat Mishra */

Python


# Python program to merge a linked list into another at
# alternate positions
class LinkedList(object):
    def __init__(self):
    # head of list
        self.head = None

    # Linked list Node
    class Node(object):
        def __init__(self, d):
            self.data = d
            self.next = None

    # Inserts a new Node at front of the list.
    def push(self, new_data):

        # 1 & 2: Allocate the Node &
        # Put in the data
        new_node = self.Node(new_data)

        # 3. Make next of new Node as head
        new_node.next = self.head

        # 4. Move the head to point to new Node
        self.head = new_node

    # Main function that inserts nodes of linked list q into p at
    # alternate positions. Since head of first list never changes
    # and head of second list/ may change, we need single pointer
    # for first list and double pointer for second list.
    def merge(self, q):
        p_curr = self.head
        q_curr = q.head

        # While there are available positions in p;
        while p_curr != None and q_curr != None:

            # Save next pointers
            p_next = p_curr.next
            q_next = q_curr.next

            # make q_curr as next of p_curr
            q_curr.next = p_next # change next pointer of q_curr
            p_curr.next = q_curr # change next pointer of p_curr

            # update current pointers for next iteration
            p_curr = p_next
            q_curr = q_next
        q.head = q_curr

    # Function to print linked list
    def printList(self):
        temp = self.head
        while temp != None:
            print str(temp.data),
            temp = temp.next
        print ''

# Driver program to test above functions
llist1 = LinkedList()
llist2 = LinkedList()
llist1.push(3)
llist1.push(2)
llist1.push(1)

print "First Linked List:"
llist1.printList()

llist2.push(8)
llist2.push(7)
llist2.push(6)
llist2.push(5)
llist2.push(4)

print "Second Linked List:"

llist2.printList()
llist1.merge(llist2)

print "Modified first linked list:"
llist1.printList()

print "Modified second linked list:"
llist2.printList()

# This code is contributed by BHAVYA JAIN


Output:
First Linked List:
1 2 3
Second Linked List:
4 5 6 7 8
Modified First Linked List:
1 4 2 5 3 6
Modified Second Linked List:
7 8 

This article is contributed by Chandra Prakash. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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