Given linked list pairs swap by changing links - c

Question

I have a singly linked list where I get the struct Node* head, and the return value is void. I'd like to swap in pairs without copying the data.
For example, if the input is 2->4->6 then the output is 4->2->6. But if the input is 2->4->6->7 then the output is 4->2->7->6.
Here is my code but it doesn't work.
void swapPairs(struct Node *head) {
struct Node *node;
while (head && head->next) {
struct Node *nxt = head->next;
head->next = nxt->next;
nxt->next = head;
node->next = nxt;
node = head;
head = node->next;
}
}

Answer 1

You need to pass the pointer to the head (or current) node by reference. Otherwise the function will deal with a copy of the value of the passed pointer. Changing the copy does not influence on the original pointer.
Here is a demonstrative program that shows how a function that swaps adjacent nodes can be written.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
struct Node
{
int data;
struct Node *next;
};
struct Node *insert( struct Node *head, size_t pos, int data )
{
if (head == NULL || pos == 0)
{
struct Node *new_node = ( struct Node * )malloc( sizeof( struct Node ) );
new_node->next = head;
new_node->data = data;
head = new_node;
}
else
{
head->next = insert( head->next, pos - 1, data );
}
return head;
}
FILE * display( const struct Node *head, FILE *fp )
{
for ( ; head != NULL; head = head->next )
{
fprintf( fp, "%d -> ", head->data );
}
fputs( "null\n", fp );
return fp;
}
void swap( struct Node **current )
{
/*
| A | -> | A | B | -> | B | C |
head
Step 1:
| B | | A | C | | B | C |
head
Step 2:
| B | | A | C | | B | A |
head
*/
// Step 1:
struct Node *tmp = *current;
*current = ( *current )->next;
tmp->next = ( *current )->next;
// Step 2:
( *current )->next = tmp;
}
void swapPairs( struct Node **head )
{
for ( ; *head && ( *head )->next; head = &( *head )->next->next)
{
swap( head );
}
}
int main()
{
struct Node *head = NULL;
const int N = 10;
srand( ( unsigned int )time( NULL ) );
for ( int i = 0; i < N; i++ )
{
head = insert( head, rand() %( i + 1 ), rand() % N );
}
fputc( '\n', display( head, stdout ) );
swapPairs( &head );
fputc( '\n', display( head, stdout ) );
}
The program output might look like
4 -> 7 -> 8 -> 1 -> 4 -> 8 -> 9 -> 0 -> 4 -> 6 -> null
7 -> 4 -> 1 -> 8 -> 8 -> 4 -> 0 -> 9 -> 6 -> 4 -> null