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.

题目：给定一个链表，交换两个邻接节点并返回。

你的算法只能使用常数空间。不能修改表中的值，只有节点自身可以改变。

思路：首先，直接交换两个节点的值。可以通过，但是不符合要求。

	public ListNode swapPairs(ListNode head) {
		if (head == null || head.next == null)
			return head;
		ListNode back = head;
		while (back != null) {
			if (back.next != null) {
				int val = back.val;
				back.val = back.next.val;
				back.next.val = val;
				back = back.next.next;
			} else
				break;
		}
		return head;
	}

	public ListNode swapPairs(ListNode head) {
		ListNode dummy = new ListNode(-1);
		dummy.next = head;
		ListNode pre = dummy;
		while (head != null && head.next != null) {
			ListNode tmp = head.next.next;
			pre.next = head.next;
			head.next.next = head;
			head.next = tmp;
			pre = head;
			head = tmp;
		}
		return dummy.next;
	}