24.c

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

structListNode {
intval;
structListNode*next;
};

structListNode*swapPairs(structListNode*head) {
if (head==NULL||head->next==NULL) returnhead;

structListNode*p=head;
structListNode*q=head->next;
structListNode*new_head=q;
structListNode*t=NULL;

while (p&&q) {
t=q->next;
q->next=p;
if (t&&t->next) {
p->next=t->next;
 }
else {
p->next=t;
break;
 }
p=t;
q=p->next;
 }

returnnew_head;
}

intmain() {
intn=5;
structListNode*head= (structListNode*)calloc(n, sizeof(structListNode));
structListNode*p=head;
inti;
for (i=0; i<n-1; i++) {
p->val=i+1;
p->next=p+1;
p=p->next;
 }
p->val=n;
p->next=NULL;

printf("Linked List: ");
p=head;
while (p) {
printf("%d ", p->val);
p=p->next;
 }
printf("\n");

p=swapPairs(head);

printf("Swap Pairs: ");
while (p) {
printf("%d ", p->val);
p=p->next;
 }
printf("\n");

return0;
}