/*
 * @lc app=leetcode id=24 lang=java
 *
 * [24] Swap Nodes in Pairs
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode prev = dummy, cur = head, next = head == null ? null : head.next, t;
        while(cur !=null && next != null) {
            prev.next = next;
            t = next.next;
            next.next = cur;
            cur.next = t;

            prev = cur;
            cur = cur.next;
            next = cur == null ? null : cur.next;
        }
        return dummy.next;
    }
}
// @lc code=end