Sorting a linked list in C

There is issues in the sortList function.

This function only put some large nodes in the beginning of the list. It is not soting all the list. you can you a sort algorithm to sort the file : quicksort/ bubblesort/... i put a code doing a sort in the end of this answer.

here is a code doing the sort of the list :

//it is replacing largest node with first one then doing the same operation with sublist (list-first element)

NodePtr sortList(NodePtr list) 
{

// 
if(list == null || list->next == null)
    return list; // the list is sorted.

//replace largest node with the first : 

//1- find largest node : 
NodePtr curr, largest,largestPrev;
curr = list;
largest = list;
prev = list;
largestPrev = list;
while(curr != NULL) {
        if(curr->num > largest->num) {
            largestPrev = prev;
            largest = curr;
        }
        prev = curr;
        curr = curr->next;

    }
//largest node is in largest. 

//2- switching firt node and largest node : 
NodePtr tmp;
if(largest != list)
{
    largestPrev->next = list;
    tmp = list->next;
    list->next = largest->next;
    largest->next = tmp;
}

// now largest is the first node of the list.

// calling the function again with the sub list :
//            list minus its first node :
largest->next = sortList(largest->next);


return largest;
}

Here is my attempt to sort a singly linked list using QuickSort algorithm. If you know n then run time will be O(n log n). Check if this helps.

#include "malloc.h"

typedef struct node {
    struct node *next;
    int val;
} node;

bool insert_node(struct node **head, int val)
{
    struct node *elem;
    elem = (struct node *)malloc(sizeof(struct node));
    if (!elem)
        return false;
    elem->val = val;
    elem->next = *head;
    *head = elem;
    return true;
}

int get_lval(struct node *head, int l)
{
    while(head && l) {
        head = head->next;
        l--;
    }
    if (head != NULL)
        return head->val;
    else
        return -1;
}

void swap(struct node *head, int i, int j)
{
    struct node *tmp = head;
    int tmpival;
    int tmpjval;
    int ti = i;
    while(tmp && i) {
        i--;
        tmp = tmp->next;
    }
    tmpival = tmp->val;
    tmp = head;
    while(tmp && j) {
        j--;
        tmp = tmp->next;
    }
    tmpjval = tmp->val;
    tmp->val = tmpival;
    tmp = head;
    i = ti;
    while(tmp && i) {
        i--;
        tmp = tmp->next;
    }
    tmp->val = tmpjval;
}


struct node *Quick_Sort_List(struct node *head, int l, int r)
{
    int i, j;
    int jval;
    int pivot;
    i = l + 1;
    if (l + 1 < r) {
        pivot = get_lval(head, l);
        printf("Pivot = %d\n", pivot);
        for (j = l + 1; j <= r; j++) {
            jval = get_lval(head, j);
            if (jval < pivot && jval != -1) {
                swap(head, i, j);
                i++;
            }
        }
        swap(head, i - 1, l);
        Quick_Sort_List(head, l, i);
        Quick_Sort_List(head, i, r);
    }
    return head;
}

struct node *Sort_linkedlist(struct node *head)
{
    struct node *tmp = head;
    // Using Quick sort.
    int n = 0;

    while (tmp) {
        n++;
        tmp = tmp->next;
    }
    printf("n = %d\n", n);
    head = Quick_Sort_List(head, 0, n);
    return head;
}

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

int _tmain(int argc, _TCHAR* argv[])
{
    struct node *head = NULL;
    struct node *shead = NULL;

    insert_node(&head, 10);
    insert_node(&head, 12);
    insert_node(&head, 9);
    insert_node(&head, 11);
    insert_node(&head, 7);
    insert_node(&head, 1);
    insert_node(&head, 3);
    insert_node(&head, 8);
    insert_node(&head, 5);
    insert_node(&head, 2);
    insert_node(&head, 4);
    insert_node(&head, 6);
    print_list(head);

    shead = Sort_linkedlist(head);
    print_list(shead);

    return 0;
}

By writing to largest->next you overwrote curr->next. So you end up restarting from the top all the time.

Make sure that:

1. the list remains consistent
2. your list iterator remains consistent

But overall, your code seems to be heavily broken, I believe there might be a couple other errors in your sorting logic.

The following are some of the problems which exist in your sorting logic:

1. You are setting the prev pointer to curr in the beginning of the loop itself which is incorrect. By doing this, you are making the current pointer and the previous node pointer as same which makes it impossible to delete the node.
2. You should assign the largest pointer also to top whereby it facilitates the possibility of setting the largest->next to real top node.

The code can modified like below (Just a pointer, you need to check for other issues yourself):

while(curr != NULL)
{

    if(curr->num > largest->num)
    {
        largest = curr;
        prev->next = curr->next;
        largest->next = top;
        top = largest;

    }
    prev = curr;
    curr = curr->next;
}

// Program to sort a single linked list in ascending order
// (without exchanging data in the nodes)
/**************************************************************************

There are two methods of sorting presented here(At a time,we can use any of
these two functions to sort our single linked list.) -

1. Function 'void Sort()' - This function uses selection sort method(I
                            think).
   In this function,a node whose data is the smallest in the list is made
   as 'head' node(i.e. starting node of the list) by scanning the whole list
   once.Then from the remaining list,again a node with the smallest data is
   found out whose address is kept in the 'next' field of previous node(head
   node).This process  continues to sort the whole list.
2. Function 'void Sort_method2()' - This function uses insertion sort
                                    method(I think).
   In this function,starting from second node in the list, all previous node
   data(starting from 'head' node) are compared with current reference node
   (which is initially second node in the list).If 'data' field of current
   reference node is smaller than that of any of its previous nodes,then
   suitable changes in the 'next' field of corresponding nodes are made.If
   data in the current reference node is smaller than that in the 'head' node,
   then the current reference node is made as 'head' node.

   *********************************************************************/

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

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

struct node *head,*head1;

void Create_node(int data);
void display();
void Sort();
void Sort_method2();


void main()
{
 int choice,d;
 clrscr();
 while(1)
 {
  printf("\n  1.Create new node");
  printf("\n  2.Sort in ascending order");
  printf("\n  3.Exit");
  printf("\nEnter your choice : ");
  scanf("%d",&choice);

   switch(choice)
   {
     case 1: printf("\nEnter data :");
             scanf("%d",&d);
             Create_node(d);
             break;
     case 2: Sort();       // At a time,we can use any of these two
             //Sort_method2();  // functions to sort our single linked list.
             break;
     case 3: exit(0);
     default:exit(0);
    }
  } // end of while(1)
 }  // end of main()

//--------------------------------------------
void Create_node(int d)
{
  struct node *newnode,*temp;
  newnode = (struct node *)malloc(sizeof(struct node));
  newnode -> data = d;
  newnode -> next = NULL;
  if(head == NULL)
     head = newnode;
  else
    {
      temp = head;
      while(temp -> next   !=   NULL)
        temp = temp -> next;

      temp -> next = newnode;
    }  // end of 'else'
}  // end of 'Create_node(int d)'

//---------------------------------------------
void display()  // Print linked list contents
{
   struct node *temp;
   printf("\nList contents are :\n");
   temp = head;
   while(temp != NULL)
   {
     printf(" Data = %d   Address = %u\n",temp->data,temp);
     temp = temp->next;
   }
   printf("\n");
}
//--------------------------------------------
void Sort()
 {
  struct node *t,*t1,*t2,*t3;
  t1 = head;
  head1 = head;
  if(head == NULL)
    printf("\nThe linked list is empty!");
  else
  {
    while( (t2 = t1 -> next)   !=   NULL)
    {
      while(t2 != NULL)
      {
        t3 = t2 -> next;
        if( t1 -> data   >   t2 -> data)
        {
          t2 -> next = t1;
          for(t = t1; t -> next != t2;t = t -> next);

          t -> next = t3;
          t1 = t2;       // t1 = Node with smaller data
          t2 = t3;       // t2 = Node to be compared with t1
        }  // end of 'if'
        else
        {
          // t1 = t1;       // That is,no change in t1.
          t2 = t3;
        }
      }  // end of ' while(t2 != NULL)'

      if(head == head1) // We want this action only for first pass of
      {                 // outer while() loop.Only initially, head = head1.
       head = t1;
       head1 = t1 -> next;
      }  // end of 'if(head == head1)'
      else
      {
        for(t = head;t -> next != head1; t = t -> next);

        t -> next = t1;
        head1 = t1 -> next;
      } // end of 'else'

      t1 = t1 -> next;
    } // end of 'while( (t2 = t1 -> next)   !=   NULL)'

    display();  // Display the list.
  }   // end of 'else' of 'if(head == NULL)'
}    // end of 'Sort()'

//--------------------------------------------
void Sort_method2()
{
 struct node *t,*t1,*t2,*tt;
 if(head == NULL)
    printf("\nThe linked list is empty!");
 else
 {
   t1 = head -> next;
   while(t1 != NULL)                         // This is i-loop(outer loop).
   {
     t2 = t1 -> next;
     for(t = head; t != t1; t = t -> next)   // This is j-loop(inner loop).
     {
       if(t1->data  <  t->data)
       {
         t1 -> next = t;
         for(tt=head; tt->next != t1; tt=tt->next); //end of for loop in 'if'

         tt -> next = t2;
         if(t == head)
           head = t1;  // There is only one statement in this 'if'.
         else  // i.e.,'if(t != head)'
         {
           for(tt=head; tt->next != t; tt=tt->next);

           tt -> next = t1;
         }
         break;
       }  // end of 'if'
     }    // end of outer 'for' loop
     t1 = t2;
   }      // end of 'while'

  display(); // Display the list.
 }        // end of 'else' of 'if(head == NULL)'
}         // end of 'Sort_method2()'