Leetcode–Swap Nodes in Pairs

The Problem:

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.

Notes:

  1. Basic question with nothing to talk about. General note for linked list: do null check before accessing node’s member.

Java Solution:

/**
 * 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) {
        ListNode fkHead = new ListNode(0);
        fkHead.next = head;
        ListNode pre = fkHead;
        ListNode p = head;
        while(p!=null && p.next!= null){
            ListNode temp = p.next.next;
            p.next.next = p;
            pre.next = p.next;
            p.next = temp;
            pre = p;
            p = p.next;
        }
        return fkHead.next;
    }
}
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