LeetCode24 成对交换节点

Given a linked list, swap every two adjacent nodes and return its head.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.
Note:

Your algorithm should use only constant extra space.
You may not modify the values in the list’s nodes, only nodes itself may be changed.
递归和迭代：

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


public ListNode swapPairs(ListNode head) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode cur = dummy;
        while(cur.next != null && cur.next.next != null){
            ListNode first = cur.next;
            ListNode second = cur.next.next;
            first.next = second.next;
            second.next = first;
            cur.next = second;
            cur = cur.next.next;
        }
        return dummy.next;
    }