[LeetCode]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.

思考：考虑几种情况：空链表，单节点链表，奇数个结点，偶数个结点。画图帮助理解。

class Solution {
public:
ListNode *swapPairs(ListNode *head) {
// IMPORTANT: Please reset any member data you declared, as
// the same Solution instance will be reused for each test case.
if(head==NULL||head->next==NULL) return head;
ListNode *p,*q,*r;
ListNode *newhead=NULL;
p=head;
q=p->next;
while(p->next)
{
p->next=q->next;
q->next=p;
if(newhead==NULL)
{
newhead=q;
r=p;
}
else
{
r->next=q;
r=p;
}
if(p->next)
{
p=p->next;
q=p->next;
}
}
return newhead;
}
};