两两交换链表中的节点（leetcode24）

输入：head = [1,2,3,4]
输出：[2,1,4,3]

public class leetcode24 {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        ListNode item4 = new ListNode(4);
        ListNode item3 = new ListNode(3,item4);
        ListNode item2 = new ListNode(2,item3);
        ListNode item1 = new ListNode(1,item2);
        
        ListNode ans = swapPairs(item1);
        ans.printNode(ans);
    }
    
    /**
     * 递归
     * 
     * @param head
     * @return
     */
    public static ListNode swapPairs(ListNode head)
    {
        if(head==null||head.next==null){
            return head;
        }
        ListNode newHead = head.next;
        head.next = swapPairs(newHead.next);
        newHead.next = head;
        
        return newHead;
    }

}

    /**
     * 迭代
     * @param head
     * @return
     */
    public static ListNode swapPairs1(ListNode head){
        ListNode dummyHead = new ListNode(0);
        dummyHead.next = head;
        ListNode temp = dummyHead;
        
        while(temp.next!=null&&temp.next.next!=null){
            ListNode node1 = temp.next;
            ListNode node2 = temp.next.next;
            temp.next = node2;
            node1.next = node2.next;
            node2.next = node1;
            temp = node1;//因为此时node1连接后面的节点
        }
        return dummyHead.next;
    }