leetcode--(24. Swap Nodes in Pairs)

描述：

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.

翻译：将单链表相邻两个节点交换位置（不是交换值）

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func swapPairs(head *ListNode) *ListNode {
    if head==nil||head.Next==nil{
        return head
    }
    r:=&ListNode{}
    res:=r
    r.Next=head
    p:=head
    q:=p.Next
    for q!=nil{
        p.Next=q.Next
        r.Next=q
        q.Next=p
        r=p
        if r==nil||r.Next==nil||r.Next.Next==nil{
            break
        }
        p=r.Next
        q=p.Next
    }
    return res.Next
}