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.
题意：把链表中的两两结点组成一对，然后调转对内的顺序。
思路：直接实现。

/**
* 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 == NULL)
return head;
ListNode *p=head, *q=NULL, *r=head;
if (p)
q = head->next;
if (p && q){
p->next = q->next;
q->next = p;
head = q;
}
r = p;
p = p->next;
q = NULL;
if (p)
q = p->next;
while (p && q){
p->next = q->next;
q->next = p;
r->next = q;
r = p;
p = p->next;
q = NULL;
if (p)
q = p->next;
}
return head;
}
};