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;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* swapPairs(ListNode* head) {
ListNode headNode(0);
ListNode* preTail = &headNode;
ListNode* nextHead = head;
while (true) {
if ((nextHead == NULL) || (nextHead->next == NULL)) {
preTail->next = nextHead;
return headNode.next;
}
ListNode* temp = nextHead->next->next;
preTail->next = nextHead->next;
nextHead->next->next = nextHead;
preTail = nextHead;
nextHead = temp;
}
}
};