Sorting and Reversing Link List
Introduction
To sort a linked list, first we traverse the list searching for the node with a minimum data value. Then we remove that node and append it to another list which is initially empty. We repeat this process with the remaining list until the list becomes empty, and at the end, we return a pointer to the beginning of the list to which all the nodes are moved, as shown in image below:
Sorting of a linked list
To reverse a list, we maintain a pointer each to the previous and the next node, then we make the link field of the current node point to the previous, make the previous equal to the current, and the current equal to the next, as shown in A linked list showing the previous, current, and next nodes at some point during reversal process.
Therefore, the code needed to reverse the list is
Prev = NULL;
While (curr != NULL)
{
Next = curr->link;
Curr -> link = prev;
Prev = curr;
Curr = next;
}
Program
# include <stdio.h>
# include <stdlib.h>
struct node
{
int data;
struct node *link;
};
struct node *insert(struct node *p, int n)
{
struct node *temp;
if(p==NULL)
{
p=(struct node *)malloc(sizeof(struct node));
if(p==NULL)
{
printf(“Error\n”);
exit(0);
}
p-> data = n;
p-> link = NULL;
}
else
{
temp = p;
while (temp-> link!= NULL)
temp = temp-> link;
temp-> link = (struct node *)malloc(sizeof(struct node));
if(temp -> link == NULL)
{
printf(“Error\n”);
exit(0);
}
temp = temp-> link;
temp-> data = n;
temp-> link = null;
}
return(p);
}
void printlist ( struct node *p )
{
printf(“The data values in the list are\n”);
while (p!= NULL)
{
printf(“%d\t”,p-> data);
p = p-> link;
}
}
/* a function to sort reverse list */
struct node *reverse(struct node *p)
{
struct node *prev, *curr;
prev = NULL;
curr = p;
while (curr != NULL)
{
p = p-> link;
curr-> link = prev;
prev = curr;
curr = p;
}
return(prev);
}
/* a function to sort a list */
struct node *sortlist(struct node *p)
{
struct node *temp1,*temp2,*min,*prev,*q;
q = NULL;
while(p != NULL)
{
prev = NULL;
min = temp1 = p;
temp2 = p -> link;
while ( temp2 != NULL )
{
if(min -> data > temp2 -> data)
{
min = temp2;
prev = temp1;
}
temp1 = temp2;
temp2 = temp2-> link;
}
if(prev == NULL)
p = min -> link;
else
prev -> link = min -> link;
min -> link = NULL;
if( q == NULL)
q = min; /* moves the node with lowest data value in the list
pointed to by p to the list
pointed to by q as a first node*/
else
{
temp1 = q;
/* traverses the list pointed to by q to get pointer to its
last node */
while( temp1 -> link != NULL)
temp1 = temp1 -> link;
temp1 -> link = min; /* moves the node with lowest data value
in the list pointed to
by p to the list pointed to by q at the end of list pointed by
q*/
}
}
return (q);
}
void main()
{
int n;
int x;
struct node *start = NULL ;
printf(“Enter the nodes to be created \n”);
scanf(“%d”,&n);
while ( n- > 0 )
{
printf( “Enter the data values to be placed in a
node\n”);
scanf(“%d”,&x);
start = insert ( start,x);
}
printf(“The created list is\n”);
printlist ( start );
start = sortlist(start);
printf(“The sorted list is\n”);
printlist ( start );
start = reverse(start);
printf(“The reversed list is\n”);
printlist ( start );
}
Explanation
The working of the sorting function on an example list is shown below
Sorting of a linked list
The working of a reverse function is shown below
Output:
. .