[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.

思路：利用两个置换，be指向前一个node，af指向后一个node

代码：

    public ListNode swapPairs(ListNode head) {
        if(head==null ||head.next==null) return head;
        ListNode node=new ListNode(0);
        node.next=head;
        ListNode temp=node,be=head,af=head.next;
        while(true)
        {
            //node置换
            temp.next=af;
            be.next=af.next;
            af.next=be;
            
            //af,be置换
            temp=af;
            af=be;
            be=temp;
            temp=af;
            //指针重置
            be=af.next;
            if(be==null)break;
            af=be.next;
            if(af==null)break;
        }
        return node.next;
    }