[LeetCode] 24. Swap Nodes in Pairs

题：https://leetcode.com/problems/swap-nodes-in-pairs/description/

题目

Given a linked list, swap every two adjacent nodes and return its head.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

Note:

1. Your algorithm should use only constant extra space.
2. You may not modify the values in the list’s nodes, only nodes itself may be changed.

题目大意

将链表中 相邻的结点 进行交换。
要求space n(1)。

思路

因为需要改变头结点所以使用了一个辅助头结点。使得代码更一致，无需 特别 处理 头结点。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode nhead = new ListNode(0);
        nhead.next = head;
        ListNode fp = nhead;
        while(fp.next!=null && fp.next.next!= null){
            ListNode sp = fp.next;
            ListNode tp = fp.next.next;
            sp.next = tp.next;
            tp.next = sp;
            fp.next = tp;
            fp = fp.next.next;
        }
        return nhead.next;
    }
}

看了 答案的写法 基本一致，现在主要的突破点是 提升 coding 速度。