leetcode(24) - Swap Nodes in Pairs

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.

/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* struct ListNode *next;
* };
*/
struct ListNode* swapPairs(struct ListNode* head) {
struct ListNode* p_1=head;

if (!p_1) return NULL;
if (!p_1->next) return head;

struct ListNode *p_2=p_1;
struct ListNode *p_3=p_1->next;
int i =0;
while(p_2 && p_3){
i++;
if(p_2==head){

p_2->next = p_3->next;
p_3->next = p_2;
head=p_3;
} else {

p_2->next = p_3->next;
p_1->next = p_3;
p_3->next = p_2;
}

p_1=p_2;

if (p_2->next != NULL) {
p_2=p_2->next;
if(p_2->next != NULL) {
p_3=p_2->next;
} else {
break;
}
} else {
break;
}

}

return head;
}