题目：
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.
思路：
保持三个指针，往前推进，不断变换
package list; public class SwapNodesInPairs { public ListNode swapPairs(ListNode head) { ListNode x = new ListNode(0); x.next = head; ListNode y = head; ListNode z = null; ListNode first = x; while (y != null && y.next != null) { z = y.next; ListNode tmp = z.next; x.next = z; z.next = y; y.next = tmp; x = y; y = tmp; } return first.next; } public static void main(String[] args) { // TODO Auto-generated method stub ListNode a1 = new ListNode(1); ListNode a2 = new ListNode(2); ListNode a3 = new ListNode(3); ListNode a4 = new ListNode(4); ListNode a5 = new ListNode(5); a1.next = a2; a2.next = a3; a3.next = a4; a4.next = a5; a5.next = null; SwapNodesInPairs s = new SwapNodesInPairs(); ListNode x = s.swapPairs(a1); while (x != null) { System.out.println(x.val); x = x.next; } } }