Given two linked lists, insert nodes of the second list into the first list at alternate positions of the 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 the second list should only be inserted when there are positions available. For example, if the first list is 1->2->3 and the second list is 4->5->6->7->8, then the first list should become 1->4->2->5->3->6 and the 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. The expected time complexity is O(n) where n is a number of nodes in the 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 implementations of this approach.
// C program to merge a linked list into // another at alternate positions #include <stdio.h> #include <stdlib.h> // A nested 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 ( "" );
} // 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 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
// Change next pointer of q_curr
q_curr->next = p_next;
// Change next pointer of p_curr
p_curr->next = q_curr;
// Update current pointers for
// next iteration
p_curr = p_next;
q_curr = q_next;
}
// Update head pointer of second list
*q = q_curr;
} // Driver code int main()
{ struct Node *p = NULL, *q = NULL;
push(&p, 3);
push(&p, 2);
push(&p, 1);
printf (
"First Linked List:" );
printList(p);
push(&q, 8);
push(&q, 7);
push(&q, 6);
push(&q, 5);
push(&q, 4);
printf (
"Second Linked List:" );
printList(q);
merge(p, &q);
printf (
"Modified First Linked List:" );
printList(p);
printf (
"Modified Second Linked List:" );
printList(q);
getchar ();
return 0;
} |
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
Time Complexity: O(N)
Auxiliary Space: O(1)
Please refer complete article on Merge a linked list into another linked list at alternate positions for more details!