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

public static ListNode swapPairs(ListNode head) {
        if (head == null || head.next == null)
            return null;
        ListNode fakehead = new ListNode(-1);
        fakehead.next = head;
        ListNode ptr1 = fakehead.next, ptr2 = fakehead.next.next, newstart = fakehead;

        while (ptr2 != null && ptr2.next != null) {
            ptr1.next = ptr2.next;
            ptr2.next = newstart.next;
            newstart.next = ptr2;
            ptr2.next = ptr1;
            newstart = newstart.next.next;
            ptr1 = newstart.next;
            ptr2 = newstart.next.next; // 最后一轮执行不到，因为判空了

        }
        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。
            ptr1.next = ptr2.next;
            ptr2.next = newstart.next;
            newstart.next = ptr2;
            ptr2.next = ptr1;
        }
        return fakehead.next;
    }