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Home Forums Write a C program to reverse a linked list in place, without using any extra memory. – MindStick
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1 Answers

Aryan Kumar
Aryan Kumar 22 Apr 2023
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#include <stdio.h>
#include <stdlib.h>
// Define the linked list node
struct ListNode {
   int val;
   struct ListNode* next;
};
// Function to create a new linked list node
struct ListNode* newNode(int data) {
   struct ListNode* node = (struct ListNode*) malloc(sizeof(struct ListNode));
   node->val = data;
   node->next = NULL;
   return node;
}
// Function to reverse a linked list in place
struct ListNode* reverseList(struct ListNode* head) {
   struct ListNode *prev = NULL, *curr = head, *next = NULL;
   while (curr != NULL) {
       next = curr->next;
       curr->next = prev;
       prev = curr;
       curr = next;
   }
   return prev;
}
int main() {
   struct ListNode* head = newNode(1);
   head->next = newNode(2);
   head->next->next = newNode(3);
   head->next->next->next = newNode(4);
   head->next->next->next->next = newNode(5);
   printf("Original List: ");
   struct ListNode* node = head;
   while (node != NULL) {
       printf("%d ", node->val);
       node = node->next;
   }
   printf("\n");
   head = reverseList(head);
   printf("Reversed List: ");
   node = head;
   while (node != NULL) {
       printf("%d ", node->val);
       node = node->next;
   }
   printf("\n");
   return 0;
}

In this program, we define the linked list node structure and a function to create a new linked list node. We then define a function reverseList that takes the head of a linked list as an argument and returns a pointer to the head of the reversed linked list. The function uses three pointers, prev, curr, and next, to keep track of the previous, current, and next nodes in the original linked list. Initially, prev is NULL, curr is the head of the linked list, and next is NULL. We traverse the linked list using a while loop and for each node, we set next to the next node, set the curr->next pointer to prev, set prev to curr, and set curr to next. Finally, we return prev, which is the head of the reversed linked list.

In the main function, we create a linked list and print the original list. We then call the reverseList function to reverse the linked list in place and print the reversed list.

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