[LeetCode]24. Swap Nodes in Pairs

题目描述：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.

解题思路1：题目要求交换相邻的链表节点。
使用递归交换相邻的两个节点，当当前节点或下一个节点为空时停止递归。

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

解题思路2：设置一个跟踪节点，每次移动两步，依次交换相邻节点。

public ListNode swapPairs2(ListNode head) {        if(head==null||head.next==null){            return head;        }        ListNode dummyNode = new ListNode(0);        dummyNode.next = head;        ListNode current = dummyNode;        while(current.next!=null&&current.next.next!=null){            ListNode first = current.next;            ListNode second = current.next.next;            first.next = second.next;            second.next = first;            current.next = second;            current = current.next.next;        }        return dummyNode.next;    }