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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