Create merging linkedlist in alternate positions.cpp

algorithms/CPlusPlus/Linked-Lists/merging linkedlist in alternate positions.cpp

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+// 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 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 = 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;
+}
+
+// This code is contributed by rathbhupendra