SwapPairs.java

    public ListNode reverseList(ListNode head) {
        if (head == null) return head;

        ListNode curr = head.next;
        head.next = null;
        while (curr != null) {
            ListNode next = curr.next;
            curr.next = head;
            head = curr;
            curr = next;
        }
        return head;
    }