Leetcode 24. Swap Nodes in Pairs

题目描述：

思路分析：

代码实现：

/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode swapPairs(ListNode head) {
if (head == null || head.next == null) {
return head;
}
ListNode newHead = new ListNode(0);
newHead.next = head;
ListNode zero = newHead;
ListNode pre = head;
ListNode p = pre.next;
ListNode next = null;
//遍历链表，每次反转两个节点
while (pre != null && p != null) {
//反转两个节点
next = p.next;
p.next = pre;
pre.next = next;
zero.next = p;
//反转两个节点之后，将指针后移
if (next == null) {
break;
} else {    //将指针后移
zero = pre;
pre = next;
p = pre.next;
}
}
return newHead.next;
}
}