24-两两交换链表中的节点

public ListNode swapPairs(ListNode head) {
    if (head == null || head.next == null) return head;
    ListNode post = head, pre = post, temp = null;
    head = head.next;
    while (pre != null && pre.next != null){
        //交换
        pre = post.next.next;
        post.next.next = post;
        //两段连接
        if (temp != null) temp.next = post.next;
        //移动到下一个位置
        temp = post;
        post = pre;
    }
    temp.next = pre;
    return head;
}

public ListNode swapPairs2(ListNode head) {
    ListNode pre = new ListNode(0);
    pre.next = head;
    ListNode temp = pre;
    while(temp.next != null && temp.next.next != null) {
        ListNode start = temp.next;
        ListNode end = temp.next.next;
        temp.next = end;
        start.next = end.next;  //这步先，就很妙，不需要方法1那样保留上一个
        end.next = start;
        temp = start;
    }
    return pre.next;
}