Given a linked list, swap every two adjacent nodes and return its head.
For example,
Given 1->2->3->4, you should return the list as 2->1->4->3.
Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.
#include#include typedef struct ListNode { int val; struct ListNode *next; }*ListNode; ListNode *swapPairs(ListNode *head) { ListNode *p1,*p2,*p3; int n=0; for(p1=p2=p3=head;p1!=NULL && p1->next!=NULL;n++){ p1=p1->next; p2->next=p1->next; p1->next=p2; if(n!=0) p3->next=p1; else head=p1; p3=p2; p1=p2=p2->next; } return head; }