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Swap Nodes in Pairs

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Description

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

Solution(Sentinel node tricky)

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/**
 * 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 pre = new ListNode(0);
        if (head == null || head.next == null) return head;
        ListNode first = head;
        ListNode second = head.next;
        pre.next = first;
        head = second;
        while (true) {
            pre.next = second;
            pre = first;
            ListNode temp = second.next;
            second.next = first;
            if (temp == null) {
                first.next = null;
                return head;
            }
            first = temp;
            second = first.next != null ? first.next : first; #Pay attention to the singular case
        }
    }
}