Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

Given

1->2->3->4

, you
should return the list as

2->1->4->3

.

挑战

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:
/**
* @param head a ListNode
* @return a ListNode
*/
ListNode* swapPairs(ListNode* head) {
// Write your code here
if (head == NULL || head->next == NULL)
{
return head;
}

ListNode *result = head->next;
ListNode *next = head->next->next;
result->next = head;
head->next = next;
ListNode *prev = head;

ListNode *p = prev->next;
ListNode *q = NULL;
while (p != NULL && p->next != NULL)
{
q = p->next;
next = q->next;
q->next = p;
p->next = next;
prev->next = q;
prev = p;
p = next;
}

return result;
}
};