Swap Nodes in Pairs LeetCode Java

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.

分析

给定一个链表，交换每两个相邻节点并返回其头。
例如，给定1 -＞2 -＞3＞4，您应该返回列表为2 -＞1＞4＞3。

代码

 1 public static ListNode swapPairs(ListNode head) {
 2         if (head == null || head.next == null)
 3             return null;
 4         ListNode fakehead = new ListNode(-1);
 5         fakehead.next = head;
 6         ListNode ptr1 = fakehead.next, ptr2 = fakehead.next.next, newstart = fakehead;
 7 
 8         while (ptr2 != null && ptr2.next != null) {
 9             ptr1.next = ptr2.next;
10             ptr2.next = newstart.next;
11             newstart.next = ptr2;
12             ptr2.next = ptr1;
13             newstart = newstart.next.next;
14             ptr1 = newstart.next;
15             ptr2 = newstart.next.next; // 最后一轮执行不到，因为判空了
16 
17         }
18         if (ptr2.next == null) { // 如果是偶数，则再交换一次。 如果是奇数就不交换了。
19             ptr1.next = ptr2.next;
20             ptr2.next = newstart.next;
21             newstart.next = ptr2;
22             ptr2.next = ptr1;
23         }
24         return fakehead.next;
25     }