LeetCode OJ:Swap Nodes in Pairs

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* reverse(ListNode *pre, ListNode *next){
ListNode *last = pre->next;
ListNode *cur = last->next;
while(cur != next){
last->next = cur->next;
cur->next = pre->next;
pre->next = cur;
cur = last->next;
}
return last;
}
ListNode *swapPairs(ListNode *head) {
ListNode *nh = new ListNode(0);
nh->next = head;
ListNode *pre = nh;
int count = 0;
while(head){
count++;
if(count % 2 ==0){
pre = reverse(pre, head->next);
head = pre->next;
}else {
head = head->next;
}
}
return nh->next;
}
};