SwapPairs_24

https://leetcode-cn.com/problems/swap-nodes-in-pairs/

image.png

（图片来源https://leetcode-cn.com/problems/swap-nodes-in-pairs/

）

notice：

1. 起点竟然是dummy

// cur 是reverse的起点
    public ListNode swapPairsN(ListNode head) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode curNode = dummy;

        while (curNode.next != null && curNode.next.next != null) {
            ListNode first = curNode.next;
            ListNode second = curNode.next.next;
            first.next = second.next;
            second.next = first;

            curNode.next = second;
            curNode = first;  // 最容易的错的地方
        }

        return dummy.next;
    }

递归

public ListNode swapPairs(ListNode head) {
        if((head == null) || (head.next == null)) {
            return head;
        }

        ListNode next = head.next;
        head.next = swapPairs(next.next);
        next.next = head;

        return next;
    }