# Reverse a sublist of linked list

• Difficulty Level : Hard
• Last Updated : 30 Dec, 2020

We are given a linked list and positions m and n. We need to reverse the linked list from position m to n.

Examples:

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```Input : 10->20->30->40->50->60->70->NULL
m = 3, n = 6
Output : 10->20->60->50->40->30->70->NULL

Input :  1->2->3->4->5->6->NULL
m = 2, n = 4
Output : 1->4->3->2->5->6->NULL```

To reverse the linked list from position m to n, we find addresses of start and end position of the linked list by running a loop, and then we unlink this part from the rest of the list and then use the normal linked list reverse function which we have earlier used for reversing the complete linked list, and use it to reverse the portion of the linked list which need to be reversed. After reversal, we again attach the portion reversed to the main list.

## C++

 `// C++ program to reverse a linked list``// from position m to position n``#include ``#include ` `// Linked list node``struct` `Node {``    ``int` `data;``    ``struct` `Node* next;``};` `// the standard reverse function used``// to reverse a linked list``struct` `Node* reverse(``struct` `Node* head)``{``    ``struct` `Node* prev = NULL;   ``    ``struct` `Node* curr = head;` `    ``while` `(curr) {``        ``struct` `Node* next = curr->next;``        ``curr->next = prev;``        ``prev = curr;``        ``curr = next;``    ``}``    ``return` `prev;``}` `// function used to reverse a linked list``// from position m to n which uses reverse``// function``Node* reverseBetween(Node* head, ``int` `m, ``int` `n)``{``    ``if` `(m == n)``        ``return` `head;` `    ``// revs and revend is start and end respectively``    ``// of the portion of the linked list which``    ``// need to be reversed. revs_prev is previous``    ``// of starting position and revend_next is next``    ``// of end of list to be reversed.``    ``Node* revs = NULL, *revs_prev = NULL;``    ``Node* revend = NULL, *revend_next = NULL;` `    ``// Find values of above pointers.``    ``int` `i = 1;``    ``Node* curr = head;``    ``while` `(curr && i <= n) {``        ``if` `(i < m)``            ``revs_prev = curr;` `        ``if` `(i == m)``            ``revs = curr;` `        ``if` `(i == n) {``            ``revend = curr;``            ``revend_next = curr->next;``        ``}` `        ``curr = curr->next;``        ``i++;``    ``}``    ``revend->next = NULL;` `    ``// Reverse linked list starting with``    ``// revs.``    ``revend = reverse(revs);` `    ``// If starting position was not head``    ``if` `(revs_prev)``        ``revs_prev->next = revend;` `    ``// If starting position was head``    ``else``        ``head = revend;` `    ``revs->next = revend_next;``    ``return` `head;``}` `void` `print(``struct` `Node* head)``{``    ``while` `(head != NULL) {``        ``printf``(``"%d "``, head->data);``        ``head = head->next;``    ``}``    ``printf``(``"\n"``);``}` `// function to add a new node at the``// beginning of the list``void` `push(``struct` `Node** head_ref, ``int` `new_data)``{``    ``struct` `Node* new_node = ``new` `Node;``    ``new_node->data = new_data;``    ``new_node->next = (*head_ref);``    ``(*head_ref) = new_node;``}` `// Driver code``int` `main()``{``    ``struct` `Node* head = NULL;``    ``push(&head, 70);``    ``push(&head, 60);``    ``push(&head, 50);``    ``push(&head, 40);``    ``push(&head, 30);``    ``push(&head, 20);``    ``push(&head, 10);``    ``reverseBetween(head, 3, 6);``    ``print(head);``    ``return` `0;``}`

## Python3

 `# Python3 program to reverse a linked list``# from position m to position n`` ` `# Linked list node``class` `Node:``    ` `    ``def` `__init__(``self``, data):``        ` `        ``self``.data ``=` `data``        ``self``.``next` `=` `None` `# The standard reverse function used``# to reverse a linked list``def` `reverse(head):` `    ``prev ``=` `None`  `    ``curr ``=` `head`` ` `    ``while` `(curr):``        ``next` `=` `curr.``next``        ``curr.``next` `=` `prev``        ``prev ``=` `curr``        ``curr ``=` `next``    ` `    ``return` `prev`` ` `# Function used to reverse a linked list``# from position m to n which uses reverse``# function``def` `reverseBetween(head, m, n):` `    ``if` `(m ``=``=` `n):``        ``return` `head``        ` `    ``# revs and revend is start and end respectively``    ``# of the portion of the linked list which``    ``# need to be reversed. revs_prev is previous``    ``# of starting position and revend_next is next``    ``# of end of list to be reversed.``    ``revs ``=` `None``    ``revs_prev ``=` `None``    ``revend ``=` `None``    ``revend_next ``=` `None`` ` `    ``# Find values of above pointers.``    ``i ``=` `1``    ``curr ``=` `head``    ` `    ``while` `(curr ``and` `i <``=` `n):``        ``if` `(i < m):``            ``revs_prev ``=` `curr`` ` `        ``if` `(i ``=``=` `m):``            ``revs ``=` `curr`` ` `        ``if` `(i ``=``=` `n):``            ``revend ``=` `curr``            ``revend_next ``=` `curr.``next`` ` `        ``curr ``=` `curr.``next``        ``i ``+``=` `1` `    ``revend.``next` `=` `None`` ` `    ``# Reverse linked list starting with``    ``# revs.``    ``revend ``=` `reverse(revs)`` ` `    ``# If starting position was not head``    ``if` `(revs_prev):``        ``revs_prev.``next` `=` `revend`` ` `    ``# If starting position was head``    ``else``:``        ``head ``=` `revend`` ` `    ``revs.``next` `=` `revend_next``    ``return` `head` `def` `prints(head):` `    ``while` `(head !``=` `None``):``        ``print``(head.data, end ``=` `' '``)``        ``head ``=` `head.``next``        ` `    ``print``()` `# Function to add a new node at the``# beginning of the list``def` `push(head_ref, new_data):` `    ``new_node ``=` `Node(new_data)``    ``new_node.data ``=` `new_data``    ``new_node.``next` `=` `(head_ref)``    ``(head_ref) ``=` `new_node``    ``return` `head_ref` `# Driver code``if` `__name__``=``=``'__main__'``:``    ` `    ``head ``=` `None``    ``head ``=` `push(head, ``70``)``    ``head ``=` `push(head, ``60``)``    ``head ``=` `push(head, ``50``)``    ``head ``=` `push(head, ``40``)``    ``head ``=` `push(head, ``30``)``    ``head ``=` `push(head, ``20``)``    ``head ``=` `push(head, ``10``)``    ` `    ``reverseBetween(head, ``3``, ``6``)``    ` `    ``prints(head)``    ` `# This code is contributed by rutvik_56`

Output:

`10 20 60 50 40 30 70`

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