Question

Nodes and linked lists in C++, how do I implement a linked list within another linked list?

Like list.add(foods(desserts(data)))

Answer 1

// C++ program to merge a linked list into another at
// alternate positions
#include <bits/stdc++.h>
using namespace std;

// A nexted list node
class Node
{
   public:
   int data;
   Node *next;
};

/* Function to insert a node at the beginning */
void push(Node ** head_ref, int new_data)
{
   Node* new_node = new 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(Node *head)
{
   Node *temp = head;
   while (temp != NULL)
   {
       cout<<temp->data<<" ";
       temp = temp->next;
   }
   cout<<endl;
}

// 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(Node *p, Node **q)
{
   Node *p_curr = p, *q_curr = *q;
   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 code
int main()
{
   Node *p = NULL, *q = NULL;
   push(&p, 3);
   push(&p, 2);
   push(&p, 1);
   cout<<"First Linked List:\n";
   printList(p);

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

   merge(p, &q);

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

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

   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 

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.