Swap Nodes in Pairs

题目描述

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.

题目解答

解题思路

该题是Reverse Nodes in K-group 特殊情况， 首先应该引入头结点，来统一化节点的操作， 用来三个节点指针(pre, first, second)来控制节点的翻转。

代码实现

public class Solution {
    public ListNode swapPairs(ListNode head) {
        if(head == null)
            return null;
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode pre = dummy;
        ListNode first = pre.next;
        ListNode second = first.next;
        while (second != null){
            //实现翻转
            first.next = second.next;
            second.next = pre.next;
            pre.next = second;

            pre =  first;
            first = pre.next;
            if(first == null)
                break;
            second = first.next;
        }
        return dummy.next;

    }
}