[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.
Idea: just go through the Linkedlist, swap the pairs.

/**
* 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) {
if(head == nullptr || head->next == NULL) return head;

ListNode *h = new ListNode(-1);
h->next = head;

ListNode *pre ;
ListNode *last;
ListNode *cur = h;
while(head != NULL && head->next != NULL){
pre = head;
last = head->next;

cur->next = last;

head = head->next->next;
last->next = pre;
pre->next = head;

cur = cur->next->next;
}

return h->next;
}
};