C Program for Folding a Linked List

March 19, 2025

C Program for Folding a Linked List

Learn how to fold a linked list

Today we learn how to write a code in C Program for fold a linked list, it is a way to divide the linked list and merge the two end of the linked list that make a fold . In below sections, we will learn steps and coe to perform the task.
For example :- Input : 10 12 3 14
Output : 10 14 12 3 (after folding linked list)

Steps required for folding a linked list :-

  • Firstly, Initialize a linked list.
  • Then find the middle point using two pointers at different speeds through the sequence of values until they both have equal values.
  •  Divide the linked list into two halves using these pointer named as slow and fast, then find middle.
  • Now reverse the second half of the linked list.
  • At last alternate merge of first and second halves.
  • Print the fold linked list.
Fold a Linked List in C 
struct node
{
int data;
struct node *next;
};

Find kth node from end of the linked list

Append the last n nodes of a linked list to the beginning of the list

Check whether linked list is palindrome or not

Print reverse of a linked list without actually reversing

Delete alternate nodes of a Linked List

Algorithm for a C Program for folding a linked list :-

  • MAKE A WHILE LOOP ( HEAD NODE 1 || HEAD NODE 2 )
  • IF ( HEAD NODE 1 )
  • THEN CURRENT →NEXT POINTER = HEAD NODE 1
  • CURRENT =CURRENT →NEXT POINTER HEAD NODE 1=HEAD NODE 1→ NEXT POINTER
  • END OF IF LOOP
  • ELSE ( HEAD NODE 2 )
  • THEN CURRENT→ NEXT POINTER = HEAD NODE 2
  • CURRENT = CURRENT→ NEXT POINTER
  • HEAD NODE 2 =HEAD NODE 2 →NEXT POINTER
  • END ELSE AND WHILE LOOP HEAD=HEAD → NEXT POINTER

C Program for folding a linked list :-

Run 
#include< stdio.h>
#include< stdlib.h>

struct node {
    int data;
    struct node* next;
};

void reverse_list(struct node** head) {
    struct node* prev = NULL, *curr = *head, *next;
    while (curr) {
        next = curr->next;
        curr->next = prev;
        prev = curr;
        curr = next;
    }

    *head = prev;
}

void create(struct node** head, int data) {
    struct node* new_node;
    new_node = (struct node*)malloc(sizeof(struct node));
    new_node->data = data;
    new_node->next = *head;
    *head = new_node;
}

void print(struct node* head) {
    struct node* ptr = head;
    while (ptr) {
        printf("%d \n", ptr->data);
        ptr = ptr->next;
    }
    printf("\n");
}

struct node* newNode(int data) {
    struct node* new_node;
    new_node = (struct node*)malloc(sizeof(struct node));
    new_node->data = data;
    new_node->next = NULL;
    return new_node;
}

void fold(struct node** head) {
    struct node* slow = *head;
    struct node* fast = slow->next;
    struct node* curr;

    while (fast && fast->next) {
        slow = slow->next;
        fast = fast->next->next;
    }

    struct node* head1 = *head;
    struct node* head2 = slow->next;
    slow->next = NULL;

    reverse_list(&head2);

    *head = newNode(0);
    curr = *head;

    while (head1 || head2) {
        if (head1) {
            curr->next = head1;
            curr = curr->next;
            head1 = head1->next;
        }
        if (head2) {
            curr->next = head2;
            curr = curr->next;
            head2 = head2->next;
        }
    }
    *head = (*head)->next;
}

int main() {
    struct node* head = NULL;
    int n, i;
    int data[100];

    printf("Enter the size of the linked list: ");
    scanf("%d", &n);
    printf("Enter data for the linked list: ");
    
    for (i = 0; i < n; i++) {
        scanf("%d", &data[i]);
    }
    
    for (i = n - 1; i >= 0; i--) {
        create(&head, data[i]);
    }
    
    printf("Linked list data: ");
    print(head);
    fold(&head);
    printf("Linked list data after folding: ");
    print(head);
    
    return 0;
}

Output :-
Enter size of link list :-
5
Enter data for link list:-
12
4
51
20
68
Link list data:-
12
4
51
20
68
Link list data after fold :-
12
68
4
20
51