[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;	
    }
};