Data Structures | Linked List | Question 5
The following function reverse() is supposed to reverse a singly linked list. There is one line missing at the end of the function.
/* Link list node */struct node{ int data; struct node* next;}; /* head_ref is a double pointer which points to head (or start) pointer of linked list */static void reverse(struct node** head_ref){ 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; } /*ADD A STATEMENT HERE*/} |
What should be added in place of “/*ADD A STATEMENT HERE*/”, so that the function correctly reverses a linked list.
(A) *head_ref = prev;
(B) *head_ref = current;
(C) *head_ref = next;
(D) *head_ref = NULL;
Answer: (A)
Explanation: *head_ref = prev;
At the end of while loop, the prev pointer points to the last node of original linked list. We need to change *head_ref so that the head pointer now starts pointing to the last node.
See the following complete running program.
#include<stdio.h>#include<stdlib.h> /* Link list node */struct node{ int data; struct node* next;}; /* Function to reverse the linked list */static void reverse(struct node** head_ref){ 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; } *head_ref = prev;} /* Function to push a node */void push(struct node** head_ref, int new_data){ /* allocate node */ struct node* new_node = (struct node*) malloc(sizeof(struct node)); /* put in the data */ new_node->data = new_data; /* link the old list off the new node */ new_node->next = (*head_ref); /* move the head to point to the new node */ (*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; }} /* Drier program to test above function*/int main(){ /* Start with the empty list */ struct node* head = NULL; push(&head, 20); push(&head, 4); push(&head, 15); push(&head, 85); printList(head); reverse(&head); printf("\n Reversed Linked list \n"); printList(head); return 0;} |
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