programming-examples/c/Search_Sorting/C Program to Implement Merge Sort Algorithm on Linked List.c

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

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

struct node* sortedmerge(struct node* a, struct node* b);
void frontbacksplit(struct node* source, struct node** frontRef, struct node** backRef);


void mergesort(struct node** headRef)
{
    struct node* head = *headRef;
    struct node* a;
    struct node* b;
    if ((head == NULL) || (head -> next == NULL))
        {
            return;
        }
    frontbacksplit(head, &a, &b);
    mergesort(&a);
    mergesort(&b);
    *headRef = sortedmerge(a, b);
}

struct node* sortedmerge(struct node* a, struct node* b)
{
    struct node* result = NULL;
    if (a == NULL)
        return(b);
    else if (b == NULL)
        return(a);
    if ( a-> data <= b -> data)
        {
            result = a;
            result -> next = sortedmerge(a -> next, b);
        }
    else
        {
            result = b;
            result -> next = sortedmerge(a, b -> next);
        }
    return(result);
}

void frontbacksplit(struct node* source, struct node** frontRef, struct node** backRef)
{
    struct node* fast;
    struct node* slow;
    if (source==NULL || source->next==NULL)
        {
            *frontRef = source;
            *backRef = NULL;
        }
    else
        {
            slow = source;
            fast = source -> next;
            while (fast != NULL)
                {
                    fast = fast -> next;
                    if (fast != NULL)
                        {
                            slow = slow -> next;
                            fast = fast -> next;
                        }
                }
            *frontRef = source;
            *backRef = slow -> next;
            slow -> next = NULL;
        }
}

void printlist(struct node *node)
{
    while(node != NULL)
        {
            printf("%d ", node -> data);
            node = node -> next;
        }
}

void push(struct node** head_ref, int new_data)
{
    struct node* new_node = (struct node*) malloc(sizeof(struct node));
    new_node -> data  = new_data;
    new_node->next = (*head_ref);
    (*head_ref) = new_node;
}
int main()
{
    struct node* a = NULL;
    push(&a, 15);
    push(&a, 10);
    push(&a, 5);
    push(&a, 20);
    push(&a, 3);
    push(&a, 26775);
    mergesort(&a);
    printf("\n Sorted Linked List is: \n");
    printlist(a);
    return 0;
}