Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

Example

Given1->2->3->4, you should return the list as2->1->4->3.

public ListNode swapPairs(ListNode head) {
    if(head == null || head.next == null) {
        return head;
    }
    ListNode dummy = new ListNode(-1);
    dummy.next = head;
    ListNode pre = dummy, p = head;
    while(p != null && p.next != null) {
        ListNode second = p.next;
        p.next = second.next;
        second.next = pre.next;
        pre.next = second;
        pre = p;
        p = p.next;
    }
    return dummy.next;
}