24. Swap Nodes in Pairs

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.

Solution 1：

 

public ListNode swapPairs(ListNode head) {
    ListNode dmy = new ListNode(0);
    ListNode cur = dmy;
    dmy.next = head;
    while (cur.next != null && cur.next.next != null) {
        ListNode first = cur.next;
        ListNode sec = first.next;
        ListNode third = sec.next;
        cur.next = sec;
        sec.next = first;
        first.next = third;
        cur = first;
    }
    return dmy.next;
}

 Solution 2：

参考题目25，k为2即可