# Reverse first K elements of given linked list

Given pointer to the head node of a linked list and a number K, the task is to reverse the first K nodes of the linked list. We need to reverse the list by changing links between nodes.
check also Reversal of a linked list

Examples:

```Input : 1->2->3->4->5->6->7->8->9->10->NULL
k = 3
Output :3->2->1->4->5->6->7->8->9->10->NULL

Input :10->18->20->25->35->NULL
k = 2
Output :18->10->20->25->35->NULL
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Explanation of the method:
suppose linked list is 1->2->3->4->5->NULL and k=3

1) Traverse the linked list till K-th point.
2) Break the linked list in to two parts from k-th point. After partition linked list will look like 1->2->3->NULL & 4->5->NULL
3) Reverse first part of the linked list leave second part as it is 3->2->1->NULL and 4->5->NULL
4) Join both the parts of the linked list, we get 3->2->1->4->5->NULL

A pictorial representation of how the algorithm works ## C++

 `// C++ program for reversal of first k elements ` `// of given linked list ` `#include ` `using` `namespace` `std; ` ` `  `/* Link list node */` `struct` `Node { ` `    ``int` `data; ` `    ``struct` `Node* next; ` `}; ` ` `  `/* Function to reverse first k elements of linked list */` `static` `void` `reverseKNodes(``struct` `Node** head_ref, ``int` `k) ` `{ ` `    ``// traverse the linked list until break ` `    ``// point not meet ` `    ``struct` `Node* temp = *head_ref; ` `    ``int` `count = 1; ` `    ``while` `(count < k) { ` `        ``temp = temp->next; ` `        ``count++; ` `    ``} ` ` `  `    ``// backup the joint point ` `    ``struct` `Node* joint_point = temp->next; ` `    ``temp->next = NULL; ``// break the list ` ` `  `    ``// reverse the list till break point ` `    ``struct` `Node* prev = NULL; ` `    ``struct` `Node* current = *head_ref; ` `    ``struct` `Node* next; ` `    ``while` `(current != NULL) { ` `        ``next = current->next; ` `        ``current->next = prev; ` `        ``prev = current; ` `        ``current = next; ` `    ``} ` ` `  `    ``// join both parts of the linked list ` `    ``// traverse the list until NULL is not ` `    ``// found ` `    ``*head_ref = prev; ` `    ``current = *head_ref; ` `    ``while` `(current->next != NULL) ` `        ``current = current->next; ` ` `  `    ``// joint both part of the list ` `    ``current->next = joint_point; ` `} ` ` `  `/* Function to push a node */` `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; ` `} ` ` `  `/* Function to print linked list */` `void` `printList(``struct` `Node* head) ` `{ ` `    ``struct` `Node* temp = head; ` `    ``while` `(temp != NULL) { ` `        ``printf``(``"%d "``, temp->data); ` `        ``temp = temp->next; ` `    ``} ` `} ` ` `  `/* Driver program to test above function*/` `int` `main() ` `{ ` `    ``// Create a linked list 1->2->3->4->5 ` `    ``struct` `Node* head = NULL; ` `    ``push(&head, 5); ` `    ``push(&head, 4); ` `    ``push(&head, 3); ` `    ``push(&head, 2); ` `    ``push(&head, 1); ` ` `  `    ``// k should be less than the ` `    ``// numbers of nodes ` `    ``int` `k = 3; ` ` `  `    ``cout << ``"\nGiven list\n"``; ` `    ``printList(head); ` ` `  `    ``reverseKNodes(&head, k); ` ` `  `    ``cout << ``"\nModified list\n"``; ` `    ``printList(head); ` ` `  `    ``return` `0; ` `} `

## Java

 `// Java program for reversal of first k elements  ` `// of given linked list  ` `class` `Sol ` `{ ` `     `  `// Link list node  ` `static` `class` `Node ` `{  ` `    ``int` `data;  ` `    ``Node next;  ` `};  ` ` `  `// Function to reverse first k elements of linked list  ` `static` `Node reverseKNodes( Node head_ref, ``int` `k)  ` `{  ` `    ``// traverse the linked list until break  ` `    ``// point not meet  ` `    ``Node temp = head_ref;  ` `    ``int` `count = ``1``;  ` `    ``while` `(count < k) ` `    ``{  ` `        ``temp = temp.next;  ` `        ``count++;  ` `    ``}  ` ` `  `    ``// backup the joint point  ` `    ``Node joint_point = temp.next;  ` `    ``temp.next = ``null``; ``// break the list  ` ` `  `    ``// reverse the list till break point  ` `    ``Node prev = ``null``;  ` `    ``Node current = head_ref;  ` `    ``Node next;  ` `    ``while` `(current != ``null``) ` `    ``{  ` `        ``next = current.next;  ` `        ``current.next = prev;  ` `        ``prev = current;  ` `        ``current = next;  ` `    ``}  ` ` `  `    ``// join both parts of the linked list  ` `    ``// traverse the list until null is not  ` `    ``// found  ` `    ``head_ref = prev;  ` `    ``current = head_ref;  ` `    ``while` `(current.next != ``null``)  ` `        ``current = current.next;  ` ` `  `    ``// joint both part of the list  ` `    ``current.next = joint_point; ` `    ``return` `head_ref; ` `}  ` ` `  `// Function to push a node  ` `static` `Node 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;  ` `    ``return` `head_ref; ` `}  ` ` `  `// Function to print linked list  ` `static` `void` `printList( Node head)  ` `{  ` `    ``Node temp = head;  ` `    ``while` `(temp != ``null``) ` `    ``{  ` `        ``System.out.printf(``"%d "``, temp.data);  ` `        ``temp = temp.next;  ` `    ``}  ` `}  ` ` `  `// Driver program to test above function ` `public` `static` `void` `main(String args[]) ` `{  ` `    ``// Create a linked list 1.2.3.4.5  ` `    ``Node head = ``null``;  ` `    ``head = push(head, ``5``);  ` `    ``head = push(head, ``4``);  ` `    ``head = push(head, ``3``);  ` `    ``head = push(head, ``2``);  ` `    ``head = push(head, ``1``);  ` ` `  `    ``// k should be less than the  ` `    ``// numbers of nodes  ` `    ``int` `k = ``3``;  ` ` `  `    ``System.out.print(``"\nGiven list\n"``);  ` `    ``printList(head);  ` ` `  `    ``head = reverseKNodes(head, k);  ` ` `  `    ``System.out.print(``"\nModified list\n"``);  ` `    ``printList(head);  ` `} ` `} ` ` `  `// This code is contributed by Arnab Kundu `

## C#

 `// C# program for reversal of first k elements  ` `// of given linked list  ` `using` `System; ` `     `  `class` `GFG ` `{ ` `     `  `// Link list node  ` `public` `class` `Node ` `{  ` `    ``public` `int` `data;  ` `    ``public` `Node next;  ` `};  ` ` `  `// Function to reverse first k elements of linked list  ` `static` `Node reverseKNodes(Node head_ref, ``int` `k)  ` `{  ` `     `  `    ``// traverse the linked list until break  ` `    ``// point not meet  ` `    ``Node temp = head_ref;  ` `    ``int` `count = 1;  ` `    ``while` `(count < k) ` `    ``{  ` `        ``temp = temp.next;  ` `        ``count++;  ` `    ``}  ` ` `  `    ``// backup the joint point  ` `    ``Node joint_point = temp.next;  ` `    ``temp.next = ``null``; ``// break the list  ` ` `  `    ``// reverse the list till break point  ` `    ``Node prev = ``null``;  ` `    ``Node current = head_ref;  ` `    ``Node next;  ` `    ``while` `(current != ``null``) ` `    ``{  ` `        ``next = current.next;  ` `        ``current.next = prev;  ` `        ``prev = current;  ` `        ``current = next;  ` `    ``}  ` ` `  `    ``// join both parts of the linked list  ` `    ``// traverse the list until null is not  ` `    ``// found  ` `    ``head_ref = prev;  ` `    ``current = head_ref;  ` `    ``while` `(current.next != ``null``)  ` `        ``current = current.next;  ` ` `  `    ``// joint both part of the list  ` `    ``current.next = joint_point; ` `    ``return` `head_ref; ` `}  ` ` `  `// Function to push a node  ` `static` `Node 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;  ` `    ``return` `head_ref; ` `}  ` ` `  `// Function to print linked list  ` `static` `void` `printList( Node head)  ` `{  ` `    ``Node temp = head;  ` `    ``while` `(temp != ``null``) ` `    ``{  ` `        ``Console.Write(``"{0} "``, temp.data);  ` `        ``temp = temp.next;  ` `    ``}  ` `}  ` ` `  `// Driver Code ` `public` `static` `void` `Main(String []args) ` `{  ` `    ``// Create a linked list 1.2.3.4.5  ` `    ``Node head = ``null``;  ` `    ``head = push(head, 5);  ` `    ``head = push(head, 4);  ` `    ``head = push(head, 3);  ` `    ``head = push(head, 2);  ` `    ``head = push(head, 1);  ` ` `  `    ``// k should be less than the  ` `    ``// numbers of nodes  ` `    ``int` `k = 3;  ` ` `  `    ``Console.Write(``"Given list\n"``);  ` `    ``printList(head);  ` ` `  `    ``head = reverseKNodes(head, k);  ` ` `  `    ``Console.Write(``"\nModified list\n"``);  ` `    ``printList(head);  ` `} ` `} ` ` `  `// This code is contributed by Princi Singh `

Output:

```Given list
1 2 3 4 5
Modified list
3 2 1 4 5 ```

Time Complexity : O(n)

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