Given a linked list, swap every two adjacent nodes and return its head.
You may not modify the values in the list’s nodes, only nodes itself may be changed.

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ListNode swapPairs(ListNode head)

由于要交换前后两个链表节点，定义一个 dummy，cur。那么核心就是交换 cur.next 和 cur.next.next。

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ListNode swapPairs(ListNode head) {
    ListNode dummy = new ListNode(0);
    dummy.next = head;
    ListNode cur = dummy;
    while(cur.next != null && cur.next.next != null) {
        ListNode a = cur.next, b = cur.next.next;
        cur.next = b;
        a.next = b.next;
        b.next = a;
        cur = a;
    }
    return dummy.next;