Reverse a sublist of linked list
We are given a linked list and positions m and n. We need to reverse the linked list from position m to n.
Examples:
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 program to reverse a linked list // from position m to position n #include <stdio.h> #include <stdlib.h> // 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 // begining 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; } |
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Output:
10 20 60 50 40 30 70
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