[Leetcode] swap nodes in pairs

Question

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 shocould use only constant space. You may not modify the values in the list, only nodes itself can be changed.

Answer

A node pointing to the header is constructed, and the header is directly traversed as a common node.

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { *         val = x; *         next = null; *     } * } */public class Solution {    public ListNode swapPairs(ListNode head) {        if(head==null||head.next==null){            return head;        }        ListNode tmp=new ListNode(0);        tmp.next=head;        ListNode p=tmp,q=head,r;        while(q!=null&&q.next!=null){            p.next=q.next;            r=q.next.next;            p.next.next=q;            q.next=r;            p=q;            q=r;        }        return tmp.next;    }}

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