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

You may not modify the values in the list's nodes, only nodes itself may be changed.

Example:

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

难度： medium

题目：

给定链表，交换相邻的两个结点并返回其头结点。不可以修改结点的值，只可以改变结点本身。

思路：加个头结点简化操作。然后就是简单的链表置换。

Runtime: 2 ms, faster than 99.98% of Java online submissions for Swap Nodes in Pairs.

Memory Usage: 26 MB, less than 6.66% of Java online submissions for Swap Nodes in Pairs.

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; } * } */class Solution {    public ListNode swapPairs(ListNode head) {        ListNode dummyHead = new ListNode(0);        dummyHead.next = head;        ListNode prev = dummyHead;        ListNode next = prev.next;        while (next != null) {            ListNode nextNext = next.next;            if (nextNext != null) {                prev.next = nextNext;                next.next = nextNext.next;                nextNext.next = next;            }            prev = next;            next = prev.next;        }                return dummyHead.next;    }}