24 swap nodes in pairs

24. 两两交换链表中的节点

go：

/**

 * Definition for singly-linked list.

 * type ListNode struct {

 *     Val int

 *     Next *ListNode

 * }
 */

func swapPairs(head *ListNode) *ListNode {
    dummy := &ListNode{Next: head}
    cur := dummy

    for cur.Next != nil && cur.Next.Next != nil {
        first := cur.Next
        second := cur.Next.Next

        first.Next = second.Next
        second.Next = first
        cur.Next = second

        cur = first
    }

    return dummy.Next
}