LeetCode_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;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *swapPairs(ListNode *head) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if(NULL == head) return head;
ListNode *pre,*p,*q;
pre = NULL; p = head; q = head->next;
int i = 0;
while(q){
if(i%2 == 0){
if(pre == NULL){
pre = q;
head = q;
}else
pre->next = q;
p->next = q->next;
q->next = p;
// move to the next;
i = 0;
q = p->next;
i++;
}else{
pre = p;
p = q;
q = q->next;
i++;
}
}
return head;
}
};